gkeyfilesettingsbackend: check for errors when creating file monitors
File monitor creation may fail. We should check for this, rather than ignoring it and then spewing criticals upon improperly assuming that we have a valid GFileMonitor rather than NULL.
In practice, creating the GFileMonitors here fail when opening a large number of tabs in Epiphany. I'm still investigating to see why, but it doesn't matter for the purposes of this commit.