Default CoreData error message is cryptic

[GoogleCodeExporter]

Take a version of a SwitchList layout file intended for a later version of 
SwitchList, and run with an earlier version.  You'll get an error like:

 "Persistent store migration failed, missing source managed object model."

That's way too cryptic; adding a more human message would be much more helpful.

There's been one case of this happening to an actual SwitchList user.

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Original issue reported on code.google.com by rwbowdi...@gmail.com on 15 Jul 2014 at 3:43