SAL Server returns custom exceptions

[TOFarmer]

Description

Enables the SAL server to respond with custom exceptions messages i.e., if a SAL exception is initialised with a message (e.g. NodeNotFound('This is a custom message'), that message will be displayed in the client.

This required:

• Adding a SALHTTPException which subclasses werkzeug.exception.HTTPException as well as SALException. This allows any classing inheriting from SALHTTPException to define a code (HTTP status code) and a description.
• Changed from using flask_restful.Api to a subclass which customises exception handling by the Api.
• Removing the error_map which was passed to flask_restful.Api.

Fixes

Fixes #46

To test

• Ensure that the exceptions which derive from SALHTTPException have the correct code values.
• Create a simple SAL server (i.e., just define a simple PersistenceProvider) and test that it can respond with custom exception messages.
• Check that standard exception messages are still returned if a custom exception message is not provided (i.e., if an exception is initialised without a message).
• Check that the changes are sufficiently documented.