Make fails cryptically if docker-squash is not installed

```
Removing intermediate container 0786e37577cb
Successfully built bfaf16cdf9ad
docker save weaveworks/scope:latest |   -t weaveworks/scope | tee scope.tar |  docker load
/bin/sh: 1: -t: not found
FATA[0000] Error response from daemon: EOF
```

Makefile should check that all dependencies exist, and produce actionable error messages if something needs to be installed.
