Improve Error Handling within API

[Jbond312]

At the moment, the services are throwing exceptions that are being caught in a generic middleware. The services are also able to specify which HTTP status code they want to return to. Whilst this works, I would prefer that the services are agnostic of any HTTP responses and that exceptions are only thrown when something exceptional happens.

Proposed changes:

• Have each command/query return an object with a base response.
  For example it could look something like:

public class BaseResult
{
	public bool Success {get; set;}
	public string Message {get; set;}
	public string Description {get; set;}
	public ValidationFailures FailureReason {get; set;}
}

public enum ValidationFailures
{
	InvalidRequest,
	Unauthorised
}

• Controllers are then responsible (as they should be) for returning the correct responses.
• The exception middleware should only then be used for unhandled exceptions