Fix issues with oauth when php and mysql timezone do not match

Summary:
- Slightly better error when /oauth/token request fails. T352
- Always use PHP time() in OAuth2_Storage_Session. T353

Test Plan:
- Set up php/mysql timezone mismatch
- Try to log in to the UI and should now succeed

Reviewers: shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T352, T353

Differential Revision: https://phabricator.ushahidi.com/D180