CiviMail: do not disable errors on test mailings

Currently we disable errors on CiviMail mailings so that one rogue recipient does not halt the whole mailing. We also disable the errors on test mailings, so that they mirror what will actually happen during a 'real' mailing.

We should disable errors only on actual mailings; having the test mailings take slightly different route, but actually report the error (and break outright) is more beneficial and allows us to debug edge cases much better.