Don't drop errors on the floor

Also report both on hmack and servack events. Although this is kinda icky.
Fixes zephyr.subscribeTo never hearing back from anything.
Provide less useless error codes

Only the zephyr ones for now. We should do more of them though. Issue #2.
Transcribe some krb5 error codes too

Man, this is annoying. Issue #2. Should write a tool that parses com_err files
and turns each into something like:

   #ifdef SOME_ERR_CODE
   NODE_ZEPHYR_ERROR(SOME_ERR_CODE)
   #endif

In case someone tries to build against an older mit-krb5 or something. Also
generate the same thing for errnos.