schema: Add error checking on remote schema files

When fetching and downloading we can get some obscure messages that are
hard to distinguish between input data errors and configuration errors.
(As seen with recent threesixtygiving test coves). This captures errors
and rewords them to be more helpful.

flattentool/schema.py

@@ -148,11 +148,27 @@ def __init__(
         if schema_filename:
             if schema_filename.startswith("http"):
                 import requests
+                import json
 
                 r = requests.get(schema_filename)
-                self.root_schema_dict = jsonref.loads(
-                    r.text, object_pairs_hook=OrderedDict
-                )
+
+                try:
+                    r.raise_for_status()
+                except requests.HTTPError:
+                    raise ValueError(
+                        _(
+                            "The URL provided for the schema in schema_filename was not accessible"
+                        )
+                    )
+
+                try:
+                    self.root_schema_dict = jsonref.loads(
+                        r.text, object_pairs_hook=OrderedDict
+                    )
+                except json.JSONDecodeError:
+                    raise ValueError(
+                        _("The schema provided in schema_filename was not valid JSON")
+                    )
             else:
                 if disable_local_refs:
                     with codecs.open(schema_filename, encoding="utf-8") as schema_file: