Fail when no Rake file exists for the subproject

[michaeljbishop]

Right now there is no special code that checks that the subproject call is passed a valid directory or Rakefile.

[michaeljbishop]

9b430bc

[michaeljbishop]

Reopening because while there is an error message, it could be better. Perhaps we could ask for an 'acknowledge' message after the server begins otherwise output a nicer message with the rake invocation for the subproject.