Add HTTP 404 handling for merge_pull_request in gitmanager.py

The code `if not response`  catches HTTP 404 as "not OK" based on the logic in the requests module. Since PR# typos are semi-common, this change pulls out the HTTP 404 to give it a different (better?) response to the user.

gitmanager.py

@@ -343,7 +343,10 @@ def merge_pull_request(cls, pr_id, comment=""):
         response = requests.get("https://api.github.com/repos/{}/pulls/{}".format(GlobalVars.bot_repo_slug, pr_id),
                                 timeout=GlobalVars.default_requests_timeout)
         if not response:
-            raise ConnectionError("Cannot connect to GitHub API")
+            if (response.status_code == 404):
+                raise ValueError("PR #{} was not found (HTTP 404) -- possible typo?".format(pr_id))
+            else:
+                raise ConnectionError("Cannot connect to GitHub API")
         pr_info = response.json()
         if pr_info["user"]["login"] != "SmokeDetector":
             raise ValueError("PR #{} is not created by me, so I can't approve it.".format(pr_id))