Clone Error instances to preserve instances of subclasses of Error

If we're dealing with an instance of a subclass of Error, we can't just make a
new Error, otherwise we lose that class information. Instead, we can clone the
original instance.
You should also assert that `err[field]` is still equal to `"email 123454@example.com failed"`.

My guess is that it's not... `deepClone` doesn't do anything for instances of a class, because it's impossible to clone a class correctly in all cases: https://github.com/Clever/underscore.deep/blob/master/underscore.deep.coffee#L36-37