Print a more appropriate message on 'rebar info'

To avoid treating plugins' info/2 functions as commands, we catch calls
to 'rebar info'. Previously (4b8c81fb) we simply printed the help
string, but it's much more informative to print the following:
Command 'info' not understood or not applicable
Merge pull request #196 from tuncer/rebar-run-info

Print a more appropriate message on 'rebar info'