Fax error codes (mpbx-fax)

Raised by the fax routes (/api/v1/fax/..., W10-FAX-01) and the fax engine supervisor.

Code	Kind	Meaning	What to do
MPBX-FAX-ENGINE-UNAVAILABLE	unavailable	The fax engine process is not configured, did not start, reported another protocol version or exited. Queued faxes wait.	Check the fax engine configuration and the server log.
MPBX-FAX-DOCUMENT-INVALID	invalid_argument	The uploaded document is not a PDF or is larger than 10 MiB.	Upload a PDF of at most 10 MiB.
MPBX-FAX-DOCUMENT-GONE	not_found	The document of this fax was removed after the retention period; the fax stays in the history.	Nothing to do; ask the sender to send it again.