[shipit] There is no suggested revision, nor error message when the last commits of a repository are all DONTBUILD

[JohanLorenzo]

It happened today with ESR68. @pascalchevrel and I were really confused. Nothing came up:

After some investigation, these lines are filtering all commits out:
https://github.com/mozilla/release-services/blob/46637c7198c64e36369e317fdab173bbaccd8609/src/shipit/frontend/src/views/NewRelease/index.js#L71-L73

I think we should show a warning explaining why nothing comes up.

What do you think @rail ?

[rail]

We definitely should warn when the list of suggestions is empty. Or maybe we can fetch more pushes in this case. Another option would be listing the DONTBUILD pushes, but make them not selectable.