better groovish reactions to bad functions.

Before:

groovy:000> show
ERROR org.codehaus.groovy.tools.shell.CommandException:
Command 'show' requires at least one argument
        at java_lang_Runnable$run.call (Unknown Source)
groovy:000> show foo
ERROR org.codehaus.groovy.tools.shell.CommandException:
Unknown function name: foo
        at java_lang_Runnable$run.call (Unknown Source)

After:

groovy:000> show
Command 'show' requires at least one argument of [variables, classes, imports, preferences, all]
groovy:000> show foo
Unknown function name: 'foo'. Valid arguments: [variables, classes, imports, preferences, all]