The prototype is actually the same for most objects. By
moving it into the internal class, we can save 8 bytes
per object, as well as allowing for some future
optimizations.

Also fix a bug in the implementation of the Error
prototype objects.

Change-Id: I4d4b641055f644a9b088f27be34bfdb0446279b7
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>