Better error for ambiguous subcommand

This could be a lot better:

```
$ dylan new
don't know how to execute subcommand {<new-subcommand>}. add an execute-subcommand method?
```

`dylan new` has no action in and of itself, but it has three subcommands, "application", "library", and "workspace". Probably just add a default method on `execute-subcommand(<subcommand>)`.