Enforce 'FCModel tables must have a single-column primary key'

I tried using `FCModel` with multiple-column tables, and after a while noticed that this isn't supposed to be supported. 

After some digging, I believe I found that `[FCModel openDatabaseAtPath: withDatabaseInitializer: schemaBuilder:]` doesn't throw an exception when it's supposed to. Say I create a 3-column key,

**Before the commit**: `isPK` returns 1 on three passes (the code looks like it's been tested on machines where SQLite returns 1, 2, 3; but I definitely get 1, 1, 1 here), `primaryKeyColumnCount ` is set to `1`, contrary to its name. No exception.

**After the commit**: `isPK` returns 1 on three passes, `primaryKeyColumnCount ` is incremented 3 times. At the end of the cycle, an exception informs me that multi-column keys are not supported.