This is a derivation of 4c1314b6, which was the same change, but for
DTMF events.  We hadn't seen a message with eventType of "error"
before, but now know that they can appear in response to certain types
of failed calls.

As in 4c1314b6, before this change jmp-fwdcalls would crash with this:

"Shutting down gateway due to exception 013: no implicit conversion of
  nil into String"

Note that the "error" messages we've seen so far contain very little
detail, so the only parameter printed out is the callState parameter.

In the long-term we may want to make our global handler a bit more
tolerant of such errors (especially ones that are difficult to
diagnose, as in this case where the error did not appear until an hour
after the call failure occurred).  But since we haven't seen many such
instances (and we're handling all the known documented cases), we will
save that for later.