Better error for ambiguous subcommand

[cgay]

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>).