Raise new HTTPError exception for HTTP errors

This enables better error reporting to client sites by sending the details of the error in separate error object fields.

The error messages themselves could still be improved quite a lot though, with some refactoring.

+ Upgrade ckanserviceprovider
+ Add HTTPError as a subclass of ckanserviceprovider's JobError,
  with custom __init__() and as_dict() methods
+ Raise HTTPError instead of JobError in various places
Fix wsgi.py

Was broken by recent refactoring.
Bump version number