Invalid specifier should return an error

[nsoblath]

Expected behavior for when a specifier is not handled should be that an error (i.e. 310) is returned. Right now that's not checked except for in Python with CMD requests, where if there's no function matching the specifier name, that error is returned.

[nsoblath]

To get a little more specific:

• endpoint: cmd's are explicitly not handled; nothing to do here
• hub: return code in cmd handling isn't right; fix to return code 310 (dl_service_error_invalid_specifier()) here:

  dripline-cpp/library/hub.cc

  Line 154 in 8c2f794

  return a_request->reply( dl_service_error_bad_payload(), "Instruction <" + t_instruction + "> not understood" );;

• service: doesn't reimplement do_cmd_request(), so nothing to do here