Bug 628937 - gracefully handle broken schemas

Implement the second feature requested in the bug: silently ignore
override files that attempt to override schemas that are not currently
installed.

Also, support 'strictness' being optional for other errors when parsing
override files (ie: inability to open the file, unknown key name, parse
errors, out of range).  We don't completely back out the file in this
case — as that is difficult with the current implementation — but just
ignore the override for the single key.