centralize handling of common errors like "endpoint not found"

Previously we
* did not distinguish between the cause of errors in mgcp_header_parse
* common errors were not handled in mgcp_handle_message() but in
  individual per-verb functions

Let's centralize the handling of generating error responses and remove
that burden from the individual per-verb handler functions.

Change-Id: I463b27306e10ae3b021583ed102977e7299e5e66
3 files changed