could use a better error message for DependencyError

devservices 1.0.6

```
josh@CTKP7QRX4L.local ~/d/sentry master $ devservices up                    
Starting 'sentry' in mode: 'default'
Retrieving dependencies
DependencyError: relay (https://github.com/getsentry/relay.git) on master
```

i think this was just a flake (retrying worked) but error message could be better here (i would also add in some retries)