Add distinct error classes

[sebgie]

This issue is an improvement to our error handling suggested by @appleYaks in #2643 (comment). We are using the following errors with the corresponding error code in api/index.js:

• BadRequest: 400
• Unauthorized: 401
• NoPermission: 403
• NotFound: 404
• RequestEntityTooLarge: 413
• ValidationError: 422
• EmailError: 500
• InternalServerError: 500

All existing API errors should be replaced by an error class that holds the corresponding error code. Internal API requests are expected to deal with the error object returned and api.requestHandler() has to convert the object into a valid JSON response.

[darvelo]

Seems you can actually tell when to catch only certain error types:

promise.then(function() {
    throw new CustomError('oops!');
}).catch(CustomError, function(e) {
    // Only catch CustomError instances
    // all other types of errors will propagate automatically
}).catch(function(e) {
    // Catch other errors
})