UX improvements for Google storage

Fix #549
Fix #552

docs/config.rst

@@ -185,6 +185,10 @@ Service to hardcode those into opensource software
 4. Finally you should have a Client ID and a Client secret. Provide these in
    your storage config.
 
+The ``token_file`` parameter should be a filepath where vdirsyncer can later
+store authentication-related data. You do not need to create the file itself
+(or write anything to it), only the folder around it.
+
 You can select which calendars to sync on `CalDav settings page
 <https://calendar.google.com/calendar/syncselect>`_.
 

vdirsyncer/storage/google.py

@@ -51,6 +51,11 @@ class GoogleSession(dav.DAVSession):
                 token = json.load(f)
         except (OSError, IOError):
             pass
+        except ValueError as e:
+            raise exceptions.UserError(
+                'Failed to load token file {}, try deleting it. '
+                'Original error: {}'.format(token_file, e)
+            )
 
         def _save_token(token):
             with atomic_write(token_file, mode='w', overwrite=True) as f: