refactor(backend): better error handling in backend (#277)

Add specific messages if Github settings were not added and change some status codes in backend

### What
Some errors raised by backend have a 500 status code intead of having 4XX error.
For example, if a developer forgets to add a Personal Access Token and wants to import a project to edit, the server throws a 500 error instead of 400 error.

### Screenshot
![image](https://github.com/openfoodfacts/taxonomy-editor/assets/106757110/e0c24f5a-9de7-47b1-a31b-20741729df8b)

### Fixes bug(s)

### Part of 
- backend/editor


---------

Co-authored-by: alice.juan <[email protected]>

backend/editor/api.py

@@ -24,6 +24,7 @@
     status,
 )
 from fastapi.encoders import jsonable_encoder
+from fastapi.exception_handlers import http_exception_handler
 from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse, JSONResponse
@@ -110,8 +111,8 @@ def file_cleanup(filepath):
     """
     try:
         os.remove(filepath)
-    except Exception as e:
-        raise HTTPException(status_code=500, detail="Taxonomy file not found for deletion") from e
+    except FileNotFoundError:
+        log.warn(f"Taxonomy file {filepath} not found for deletion")
 
 
 class StatusFilter(str, Enum):
@@ -139,6 +140,16 @@ async def validation_exception_handler(request: Request, exc: RequestValidationE
     )
 
 
+@app.exception_handler(HTTPException)
+async def log_http_exception(request: Request, exc: HTTPException):
+    """
+    Custom exception handler to log FastAPI exceptions.
+    """
+    # Log the detail message
+    log.info(f" ERROR: {exc.detail}")
+    return await http_exception_handler(request, exc)
+
+
 # Get methods
 
 
@@ -351,10 +362,10 @@ async def export_to_github(
         return url
 
     except GithubBranchExistsError:
-        raise HTTPException(status_code=500, detail="The Github branch already exists!")
+        raise HTTPException(status_code=409, detail="The Github branch already exists!")
 
     except GithubUploadError:
-        raise HTTPException(status_code=500, detail="Github upload error!")
+        raise HTTPException(status_code=400, detail="Github upload error!")
 
 
 # Post methods
@@ -370,7 +381,7 @@ async def import_from_github(request: Request, branch: str, taxonomy_name: str):
 
     taxonomy = TaxonomyGraph(branch, taxonomy_name)
     if not taxonomy.is_valid_branch_name():
-        raise HTTPException(status_code=400, detail="branch_name: Enter a valid branch name!")
+        raise HTTPException(status_code=422, detail="branch_name: Enter a valid branch name!")
     if await taxonomy.does_project_exist():
         raise HTTPException(status_code=409, detail="Project already exists!")
     if not await taxonomy.is_branch_unique():
@@ -390,7 +401,7 @@ async def upload_taxonomy(
     # use the file name as the taxonomy name
     taxonomy = TaxonomyGraph(branch, taxonomy_name)
     if not taxonomy.is_valid_branch_name():
-        raise HTTPException(status_code=400, detail="branch_name: Enter a valid branch name!")
+        raise HTTPException(status_code=422, detail="branch_name: Enter a valid branch name!")
     if await taxonomy.does_project_exist():
         raise HTTPException(status_code=409, detail="Project already exists!")
     if not await taxonomy.is_branch_unique():

backend/editor/github_functions.py

@@ -3,7 +3,8 @@
 """
 from textwrap import dedent
 
-from github import Github
+from fastapi import HTTPException
+from github import Github, GithubException
 
 from . import settings
 
@@ -21,8 +22,19 @@ def init_driver_and_repo(self):
         """
         Initalize connection to Github with an access token
         """
-        github_driver = Github(settings.access_token)
-        repo = github_driver.get_repo(settings.repo_uri)
+        access_token = settings.access_token
+        if not access_token:
+            raise HTTPException(
+                status_code=400,
+                detail="Access token is not set. Please add your access token in .env",
+            )
+        repo_uri = settings.repo_uri
+        if not repo_uri:
+            raise HTTPException(
+                status_code=400, detail="repo_uri is not set. Please add your access token in .env"
+            )
+        github_driver = Github(access_token)
+        repo = github_driver.get_repo(repo_uri)
         return repo
 
     def list_all_branches(self):
@@ -47,19 +59,28 @@ def update_file(self, filename):
         # Find taxonomy text file to be updated
         github_filepath = f"taxonomies/{self.taxonomy_name}.txt"
         commit_message = f"Update {self.taxonomy_name}.txt"
+        try:
+            current_file = self.repo.get_contents(github_filepath)
+            with open(filename, "r") as f:
+                new_file_contents = f.read()
 
-        current_file = self.repo.get_contents(github_filepath)
-        with open(filename, "r") as f:
-            new_file_contents = f.read()
-
-        # Update the file
-        self.repo.update_file(
-            github_filepath,
-            commit_message,
-            new_file_contents,
-            current_file.sha,
-            branch=self.branch_name,
-        )
+            # Update the file
+            self.repo.update_file(
+                github_filepath,
+                commit_message,
+                new_file_contents,
+                current_file.sha,
+                branch=self.branch_name,
+            )
+        except GithubException as e:
+            # Handle GitHub API-related exceptions
+            raise Exception(f"GitHub API error: {e}") from e
+        except FileNotFoundError as e:
+            # Handle file not found error (e.g., when 'filename' does not exist)
+            raise Exception(f"File not found: {filename}") from e
+        except Exception as e:
+            # Handle any other unexpected exceptions
+            raise Exception(f"An error occurred: {e}") from e
 
     def create_pr(self, description):
         """