Trying to grab a hotkey that is already grabbed fails silently

While implementing keyboard navigation in the launcher, I was trying to register a Hotkey watch for Alt+F1 to give the keyboard focus to the launcher. But the slot I connected to the pressed() signal would never be triggered.

We later discovered that this was because the Alt+F1 combination was already grabbed by metacity. Trying to grab a key combination that is already grabbed normally results in a BadAccess X error, but this error was swallowed by the error handler set up in Unity2dPlugin::initializeEngine(…). Note that there is already a FIXME in the code to improve this error handler:

/* FIXME: This should be done more properly, it's just an hack this way.
          We should silence only the errors that we know we can produce.
          We could probably also learn something from gdk-error-trap-push.
          See: http://library.gnome.org/devel/gdk/stable/gdk-General.html#gdk-error-trap-push
 */