Show nicer error message if allocating the MFA setup fails

Show nicer error message if allocating the MFA setup fails

`Setup::allocateSetUpId` can deadlock if the form is submitted twice at the
same time. This error should not be normally seen by the user. If they do they
will be directed to "Try again" and then see that MFA is active, because one of
the requests succeeded. They will also receive the info mail letting them know
where to regenerate their backup codes if necessary.