g_thread_new: never fail

Remove the GError argument from g_thread_new() and abort on failure.
Introduce g_thread_try() for those who want to handle failure.

gio/tests/gdbus-proxy-threads.c

@@ -222,9 +222,7 @@ test_proxy (void)
   for (i = 0; i < n_threads; i++)
     {
       proxy_threads[i] = g_thread_new ("run-proxy",
-                                       run_proxy_thread, connection,
-                                       &error);
-      g_assert_no_error (error);
+                                       run_proxy_thread, connection);
     }
 
   g_main_loop_run (loop);