Check for failures in provisioning after returning from an IdP.

* This is a standin for checking for disabled 3rd party cookies. If we expect
provisioning to succeed, but it fails, this is probably due to disabled 3rd party cookies.

fixes #3520
Update text with @ryanfeeley's suggestions.

Thanks @ryanfeeley!
Update with @seanmonstar's suggestions for escaping.
Merge pull request #3690 from mozilla/issue_3520-3p_cookies_disabled

Check for failures in provisioning after returning from an IdP.