Ubiquity import error detection improvements, NSMetadataQuery fixes, PSC locking.

[IMPROVED]  Ubiquity import error detection: Detect validation errors buried in NSDetailedErrors.
[RENAMED]   Notifications renamed to consistently prefix symbols with USM.
[UPDATED]   Docs of willLoad updated with new use recommendations: do a save and unset in performBlockAndWait:.
[FIXED]     Now we can do willLoad before removing the stores again.
[UPDATED]   Cleaned up file presenter handling.
[FIXED]     PSC locking: wasLocked test didn't work.
[IMPROVED]  Scope the PSC locking over more work.
[IMPROVED]  Error detection while writing UUID files.
[FIXED]     Don't import ubiquitous changes with a custom MOC: there's no point and it might fail because willLoad doesn't block on it.
[FIXED]     NSMetadataQuery monitoring: fix predicate, scope notification observer to query and START THE QUERY ON THE MAIN THREAD
            BECAUSE IT SILENTLY DOES NOTHING WHEN THERE'S NO RUN LOOP ON THE STARTING THREAD.  Apple should really document this better.