Email already registered error shown, but user still added to correct group

[FinHorsley]

If the email address entered is already registered with TestFlight, but not added to the group(s) specified by the ITC_APP_TESTER_GROUPS var, the Email Already registered error is shown. Despite the error, the email is still added to the correct group(s).

If the user is already part of TestFlight, but hasn't been added to the groups specified, the error should not be shown. Instead the user should just be added to group(s), and an success message should appear.

This may well be related to #188