Improve exception handling on reader account login

- If there's an issue when logging in (e.g. due to invalid credentials), the user will now return a 500, not a 403

datagateway_api/src/datagateway_api/icat/reader_query_handler.py

@@ -13,7 +13,7 @@ class ReaderQueryHandler:
     # TODO - better names and comments on dicts to explain what they're for
     # TODO - add docstrings
     entity_filter_check = {"Datafile": "dataset.id", "Dataset": "investigation.id"}
-    entity_type_check = {"Datafile": "Dataset", "Dataset": "Investigation"}
+    parent_entity_lookup = {"Datafile": "Dataset", "Dataset": "Investigation"}
     # keep a cached reader_client for faster queries
     reader_client = None
 
@@ -36,10 +36,13 @@ def create_reader_client(self):
             "username": reader_config.reader_username,
             "password": reader_config.reader_password,
         }
-        ReaderQueryHandler.reader_client.login(
-            reader_config.reader_mechanism,
-            login_credentals,
-        )
+        try:
+            ReaderQueryHandler.reader_client.login(
+                reader_config.reader_mechanism, login_credentals,
+            )
+        except ICATSessionError:
+            log.error("User credentials for reader account aren't valid")
+            raise PythonICATError("Internal error with reader account configuration")
         return ReaderQueryHandler.reader_client
 
     def check_eligibility(self):
@@ -77,8 +80,7 @@ def is_user_authorised_to_see_entity_id(self, client):
             self.where_filter_entity_id,
         )
         access_query = ICATQuery(
-            client,
-            ReaderQueryHandler.entity_type_check[self.entity_type],
+            client, ReaderQueryHandler.parent_entity_lookup[self.entity_type],
         )
         id_check = PythonICATWhereFilter("id", self.where_filter_entity_id, "eq")
         access_filter_handler = FilterOrderHandler()