dig should use a non-zero exit code when an AXFR query fails

Attempting an AXFR using dig I received a truncated AXFR:

;; communications error to x.x.x.x#53: end of file

It's currently impossible to detect this other than by parsing the file looking for the error or the missing SOA record.

It would be more useful if dig exited with a non-zero status code when this happens.