Add hooks to customize error output

Errors generated by serializer classes can be customized through methods that the user can override in its serializer subclasses.

I did not like the way errors were built when deserializing objects using the `many` argument, having many empty dicts in the response content if only one of the 40 objects I POSTed was wrong. Instead, I wanted errors to be nested under a key taken from one of the attributes of the malformed object.

I think my implementation needs refining though, but could this be an interesting feature to get into the core of rest_framework?