Use "UNKNOWN" when no tags are present

On repository forks and similar, often no tags
are present leading to travis failures. This now
logs that there is no tag present for `git describe`
and returns "UNKNOWN". This may cause subsequent
code to fail, but those repositories should simply add a
fake tag if it expects a given format:

```
   git describe || git tag -m "fake" v0.0.0
```

This will need to be released as 0.1.0 after which I can bump `omego`.

/cc @sbesson @manics
See https://github.com/openmicroscopy/yaclifw/pull/3