#209: Change ICATSessionError exception to output exception message

- When running the API, it was difficult to distinguish between the AuthenticationError affected in this commit, and the one a couple of lines above it

datagateway_api/common/icat/helpers.py

@@ -66,8 +66,8 @@ def wrapper_requires_session(*args, **kwargs):
                 raise AuthenticationError("Forbidden")
             else:
                 return method(*args, **kwargs)
-        except ICATSessionError:
-            raise AuthenticationError("Forbidden")
+        except ICATSessionError as e:
+            raise AuthenticationError(e)
 
     return wrapper_requires_session