chan_pjsip.c: Improve ast_request() diagnostic msgs.

Attempting to dial PJSIP/endpoint when the endpoint doesn't exist and
disable_multi_domain=no results in a misleading empty endpoint name
message.  The message should say the endpoint was not found.

* Added missing endpoint not found message.

* Added more information to the empty endpoint name msgs if available.

* Eliminated RAII_VAR in request().

Change-Id: I21da85ebd62dcc32115b2ffcb5157416ebae51e4