Don't sign out if can't migrate core data model

After the core data changes, when a persistent store was not compatible,
the following sequence of events happened:

- Core data starts initialization when [ContextManager mainContext] is
  called
- Trough a dispatch_once it tries to setup the PSC
- The PSC setup fails, deletes the file and calls signOut
- Somewhere along the stack trace, signOut calls [ContextManager
  mainContext] again

And this would deadlock waiting for the previous dispatch_once to return while inside it.

Since credentials are (or will be) managed by core data WPAccount,
there's no reason to call WordPressComApi here anymore

refs #22