res_parking: Fail gracefully if parking lot is full.

res_parking: Fail gracefully if parking lot is full.

Currently, if a parking lot is full, bridge setup returns -1,
causing dialplan execution to terminate without TryExec.
However, such failures should be handled more gracefully,
the same way they are on other paths, as indicated by the
module's author, here:

http://lists.digium.com/pipermail/asterisk-dev/2018-December/077144.html

Now, callers will hear the parking failure announcement, and dialplan
will continue, which is consistent with existing failure modes.

Resolves: #624