res_pjsip_t38: Decline T.38 stream on failure case.

When negotiating an incoming T.38 stream the code incorrectly
returned failure instead of a decline for the stream when a
problem occurred or the configuration didn't allow it. This
resulted in SDP offers being rejected with a 488 response
in all cases, even when another valid stream was present.

This change makes it so the stream is now declined. If no
streams are accepted a 488 response is sent while if at least
one stream is accepted all the declined streams are, well,
declined.

ASTERISK-27763

Change-Id: I88bcf793788c412a9839d111a5c736bf6867807c