Preventing errors in a new repo with no branches or remotes

I tried legit for the first time and it was natural to me to do
```
mcd /tmp/wut
git init
legit install
```

I got some problems:
- With legit from pip ```legit install``` worked well but then a simple command like ```git branches``` failed hard with an ugly exception (I added an error message for that)
- With legit from your repo ```legit install``` raised an exception as there were no remotes and there's a piece of code in scm.py that calls ```remote = get_remote()```. I added a fake remote without pulling or fetching, ```git branches``` failed like in the previous point for an ```AssertionError``` because of ```for b in remote.refs:```

I fixed those problems adding a meaningful error message