Provide better error messages when a vue.js component fails to load its API data

[zackgalbreath]

Bug report

Expected Behavior

Make it apparent what the problem is when APP_URL is misconfigured.

Actual Behavior

Typically a blank page is returned & you need to open the developer console to see that the API call failed.

CDash Version

6c71624b12f2a07546de20354b196f9ce6e30ea3