Respond to failed deployments

**Is your feature request related to a problem? Please describe.**
Hey! Netlify-CMS has been great so far, thanks! One thing i've found when working with the content authors is if the compile fails there is no way for a content author to know, it just shows Check for preview forever.

**Describe the solution you'd like**
When the GitHub status is failed, we should show some message instead of Check for preview.


As an additional suggestion, but more opinion based, I think the current UI is a little unclear. You click Check for preview and nothing seems to happen. _I_ know that it sends a request to check the status, but I think for the layperson it's a bit less clear. Might it be better for it to say Building preview..., or similar - we could still have a little refresh icon too to check for updates. My site takes about 5 minutes to compile, so I think it feels a bit mysterious for content authors. Even a little visual feedback from spinning the refresh icon might help, just to show something happening.

I don't mind having a look at these issues, but I wanted to test the water and check you're interested and I wasn't missing something first 😅