player: actually report an exit error if encoding mode fails on closing

The code that determines the process exit code ignores all stop_play
values other than PT_QUIT. Generally, PT_ERROR is meaningless outside of
play_current_file(), and is mostly equivalent to PT_NEXT_ENTRY.

Do something that makes it report a non-0 exit code, and indicates in
the terminal exit message that something went wrong.

Untested.

player/loadfile.c

@@ -1809,7 +1809,7 @@ void mp_play_files(struct MPContext *mpctx)
         uninit_video_out(mpctx);
 
         if (!encode_lavc_free(mpctx->encode_lavc_ctx))
-            mpctx->stop_play = PT_ERROR;
+            mpctx->files_errored += 1;
 
         mpctx->encode_lavc_ctx = NULL;
     }