serializers: review errors.py and decouple error abstraction from serialization

[ppanero]

Error classes deal with abstracting a representation of the errors in the Flask views, so that they can be serialized in a second phase.

Right now they are coupled to a JSON serialization (in the class itself). However, this should be done by the make_error_response function in the resources.py (per resource class). Serializing according to the accepted mimetype.

Open Questions:

• Should the error object implement Serializable? Should it be a different mixin "SerialiableError"?
• How does the serialize_error and the make_error_response play with abort, application errors and handlers.