Error standardization #15
Description
Some methods in the SQL subspec context, mainly those which modify data in the database, create and return NSError objects.
Those errors have as a description the database error message, as the domain com.CopyIsRight.GoldDigger and as the code... well, just a random number. As you may see in #14
So, to follow the right patterns, the possible error codes should be understood and all the returned combinations should be external constants describing what kind of error that code is related to.