Better feedback for invalid UUID during model update

When specifying a UUID that is too long or has invalid characters, a server side error 500 is returned:

Instead the toolkit should do client side validation of the UUID in the model.yaml (as it currently does for many other portions of the yaml content) and present a sensical error to the user, like:

App UUID's cannot exceed 23 characters, the App UUID in the model.yaml supplied is 51 characters.

or

App UUID's cannot lower case characters or non-alphanumeric characters, the App UUID in the model.yaml contains a lowercase character.

Where '23' and 'lower case characters or non-alphanumeric characters' are just examples. A full description of the limitations should be investigated and the validation should cover all of them.