Page refresh after Linking external service should not end up as crash

Steps to reproduce:

1. Set up OAuth with Google
2. Open user control panel / Profile / Manage external account associations
3. Click on Link in Google section and confirm (Allow) login via Google
4. Press F5 (or ctrl+R depending on your browser)
5. phpBB will repeat the request and will end up with ugly error message output
6. Press F5 (or ctrl+R depending on your browser)
7. phpBB will show its standard fatal error dialog explaining that unique key violation occured (because there already is a Google account registered)

Expected behaviour:
Simply inform the user that the account has already been linked