res_xmpp: Try to provide useful errors messages from OpenSSL

If any errors occur during the TLS connection setup, we currently dump a
fairly generic error message. So instead we try to pull in something
useful from OpenSSL to report instead.

ASTERISK-24712
Reported by: Matthias Urlichs

Change-Id: I288500991a9681f447d92913b11fedaf426087f4