Error handling verification email (social) sign-up

Descriptive summary
This addresses a bug (missing error handling) during (social) sign-up in combination with sending verification emails, eg. (third party) email sever issue.

Rationale
There is no reason the user is presented a 500 since he was signed up correctly and DEFAULT ACCOUNT_EMAIL_VERIFICATION (="optional"). The latter case ACCOUNT_EMAIL_VERIFICATION (="mandatory") will require separate attention.

.Actual behavior
When verification email can't be send, e.g. email sever problem, the new user is presented a 500 page. However, the user was correctly signed-up and is signed in, i.e. has access to /projects .

Expected behavior
When verification email can't be send, e.g. email sever problem, the new user is automatically directed to /projects page, since he has been signed in anyway.
In case of ACCOUNT_EMAIL_VERIFICATION (="mandatory") a error page is presented including contact to the service providers.

Related work

Start by tracing the social sign-up flow and its verification-email handling, using ACCOUNT_EMAIL_VERIFICATION and the /projects redirect described in the issue. Verify the optional case keeps a successfully signed-in user out of the 500 page, while the mandatory case presents an appropriate error page with service-provider contact information.