brew.rb: adjust behavior for undocumented commands

Don't exit with a non-zero status code just because an internal command
happens to be undocumented. Also print the generic help in these cases
to provide some help (including a reference to the man page).

Library/brew.rb

@@ -83,8 +83,9 @@ begin
       if help_text.nil?
         # External command, let it handle help by itself
       elsif help_text.empty?
-        puts "No help available for #{cmd}"
-        exit 1
+        opoo "No help available for '#{cmd}' command."
+        puts ARGV.usage
+        exit 0
       else
         puts help_text
         exit 0