Enable custom exception messages to be raised from SAL Server

[TOFarmer]

Description

The Flask implementation of the SAL server relies on all exception types (and their messages) being defined when the server is initialised. This means that context specific messages (i.e. origin of the exception) cannot be returned to the client.

Suggested fix

Remove the server error_map and subclass flask_restful.Api to provide the ability to return custom exception messages.

Additional context

Although commonly a generic exception message (i.e. NodeNotFound: The supplied path does not point to a valid node.) will be sufficient for the client, occasionally additional context will be beneficial. For example, if the SAL server is interacting with a backend with a large number of exception types, there will be a many-to-one mapping to SAL exception types; in this instance being able to communicate the specific backend error message to the user could be useful.

[TOFarmer]

Another example of when this would be useful already exists when using POST:

• If invalid content (i.e., content which cannot be deserialised within SAL) is sent as the JSON when using POST, an InvalidRequest('Could not de-serialise content.') is raised.
• However if the content is valid but is not a Branch or DataObject, the InvalidRequest message is 'Content does not describe a Branch or DataObject'.

In the current handling of exception neither or these messages would be passed to the user.