Catching errors in requiring gmail.

https://virtru.atlassian.net/browse/BP-878

To test:

Open gmail in basic HTML mode and make sure you get the popup explaining that the mode isn't supported.
To open gmail in basic HTML mode, open gmail.com, and really quick while it's loading, hit the 'basic HTML view' link in the bottom right hand corner.

Do the same in Yahoo, only to switch it to basic mode in yahoo, just go to their settings page.

I'm not aware of a basic mode in Outlook.