[zfcampus/zf-oauth2#19] Added tests for new behavior

- Idea is to raise an exception if an invalid DSN is provided
[zfcampus/zf-oauth2#19] Fix for issue

- Added code in both create() and update() to validate the DSN, and raise an
  exception immediately if the DSN is invalid.
[zfcampus/zf-oauth2#19] Add error messages to UI

- If an error occurs during create/update, flash an error message.