Improved error handling

[seanjparker]

The error handling in hs_applications is very basic right now. We don't have a 404 page and the ApiErrors are chosen using a switch statement which could be made much cleaner using a map (see here)

Also, we should define some proper error codes since we just throw errors which messages so replacing this with an Enum would be a nice addition. Another nice feature would be wrapping errors, currently when an error occurs in a async function the original error message is dropped.