res_pjsip: Change log message from error to warning for valid use cases

If a SIP MESSAGE is triggered for an endpoint that is currently not registered
- and therefore has no valid contact associated - an error message was logged.
Since this is a valid request in a valid use cases this is now changed to a
warning, as discussed with Matt Fredrickson on the asterisk-dev mailing list.

Change-Id: I55eb62d2712818a58c7532119dec288bd98cf0c0