Avoid creating the NSMutableArray of errors until we actually get one.  This gets called for every attribute mapped, and the creation was actually pretty expensive overall.  Also use +new to avoid the immediate autorelease (will be done by ARC if eventually returned), and avoided creation of the error userInfo if the error parameter was NULL.  The array instance was the imporant one though.
Merge pull request #15 from carllindberg/2065-AvoidErrorsArray

Avoid errors NSArray creation until error actually happens