`error: store.RevisionNotAvailable with 2 snaps` when installing two snaps

[Dan Watkins]

Whilst trying to reproduce another issue (bug 1808048), I was attempting to reinstall the two snaps I'd previously seen cause the issue:

```
$ snap install anbox canonical-livepatch
error: store.RevisionNotAvailable with 2 snaps
```

This messaging isn't all that great. I'm _pretty sure_ it's because anbox doesn't have a revision in the stable channel yet, but the error message obviously does nothing to guide me towards that. This could definitely be better.

[John Lenton (chipaca)]

$ snap install anbox canonical-livepatch
error: cannot install "anbox", "canonical-livepatch": cannot install snap "anbox": no snap revision
       available as specified

[John Lenton (chipaca)]

Thinking about this some more, I'll see if we can't make it even better.