Service Discovery: Improve visual feedback in New Service page

Improve the error messages shown to the user when a request fails or it's empty:

Now we only show a generic message like: "Cannot read property 'X' of undefined"

So the ruby server responds with a 200 on the request: "Give me the list of services that are discoverable"
When there is none we should send a 404 or if there is another error due to another reason send a 50x and not 200 and an empty list

On Javascript side it should be handled according to the response.

Disable user interaction when there's an AJAX request ongoing:
Disable form when a request is in process and show a spinner to make user aware of this.