Better error messages

```
~ gg clone
thread 'main' panicked at 'Failed to clone repo https://github.com/golang/net.git, : Error { code: -4, klass: 3, message: "\'blah/blah/net\' exists and is not an empty directory" }', src/libcore/result.rs:1165:5
```
Clone fails with stacktrace when the local path it tries to clone into already exists. Handle the error with a better message