Help registry

Command line apps like tap could really benefit from a help registry so that certain errors can be supplemented with user help.

begin
  # ...
rescue
  raise if $DEBUG
  puts $!.message

  # replace this with a help registry
  if $!.message == "no nodes specified" && !ARGV.empty?
    puts "(did you mean 'tap run -- #{ARGV.join(' ')}'?)"
  end
  exit(1)
end

exit(0)