gst/playback/gstplaybasebin.c: Catch async errors when starting up the subtitle bin, so we can stop waiting and conti...

Original commit message from CVS:
* gst/playback/gstplaybasebin.c: (subbin_startup_sync_msg),
(setup_source):
Catch async errors when starting up the subtitle bin, so we can
stop waiting and continue with the main film instead of hanging
forever. Fixes #339366.
* tests/check/elements/playbin.c: (playbin_suite):
Enable unit test for the above.