Improve error reporting with oauth login strategies

[knolleary]

Proposed changes

Previously, if there was a problem with logging in using a Passport strategy (rather than username/password), there were two possible results:

1. the page reloads to show the login dialog with no visible feedback there was an issue
2. a blank page containing a stack trace from the internals of Passport

With this PR, we now always redirect back to the login dialog and also provide feedback that the login failed. The feedback will be the error description returned by the passport strategy. In the case of Oauth, that'll be the error_description field in the oauth response.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality