Abort with friendly message if no command specified

[kofalt]

Running hroot with an invalid target such as hroot build DoesNotExist or other such problems can result in a configuration with no command. Right now the result is that docker crashes, then so do we.

We should be smarter about detecting configuration results that will cause docker to fail anyway, and abort before trying - ideally before even connecting to docker.

A friendlier message will make these kind of config problems easier to discover & solve.