gst-launch: Fix process return value on error

In case of a run-time error message, the process return value was left
unset. This would lead to error not being caught at shell level.

https://bugzilla.gnome.org/show_bug.cgi?id=759019

tools/gst-launch.c

@@ -1111,6 +1111,7 @@ main (int argc, char *argv[])
 
       tfthen = gst_util_get_timestamp ();
       caught_error = event_loop (pipeline, TRUE, FALSE, GST_STATE_PLAYING);
+      res = caught_error;
       if (eos_on_shutdown && caught_error != ELR_NO_ERROR) {
         gboolean ignore_errors;