Revert "GSettings: don't abort on missing schemas"

Revert "GSettings: don't abort on missing schemas"

This reverts commit c841c2ce3fda6f754c88ae2c9099f36dff2f0814.

This approach has been an unmitigated disaster.  We're getting all sorts
of crashes due to functions that are returning NULL because they can't
find the schema for the default value.  The people who get these crashes
are then confused about the root cause of the problem and waste a lot of
time trying to figure it out.

Until we find a better solution, we should go back to what we had
before.

https://bugzilla.gnome.org/show_bug.cgi?id=655366