Which also means that it is now producing *something* for any base64
payload, which is what leads to the couple of older test changes in
test_email.  This is a slightly backward incompatible behavior change,
but the new behavior is so much more useful than the old (you can now
*reliably* detect errors, and any program that was detecting errors by
sniffing for a base64 return from get_payload(decode=True) and then doing
its own error-recovery decode will just get the error-recovery decode
right away).  So this seems to me to be worth the small risk inherent
in this behavior change.

This patch also refactors the defect tests into a separate test file,
since they are no longer just parser tests.