gpg: Make sure a DECRYPTION_OKAY is never issued for a bad OCB tag.

* g10/mainproc.c (proc_encrypted): Force a decryption failure if any
error has been seen.
* g10/decrypt-data.c (aead_checktag): Issue an ERROR line.

Note that gpg in any case returns a failure exit code but due to
double forking GPGME would not see it.

• GnuPG-bug-id: T7042