Do not print a backtrace for SystemExit

We throw this whenever `exit` is used so it's not an uncaught error but just a
quirk of ruby.

Author: carlosmn

bin/cm

@@ -35,8 +35,10 @@ pre { |global,command,options,args| true }
 post { |global,command,options,args| true }
 
 on_error do |exception|
-  puts exception
-  puts exception.backtrace
+  if !exception.is_a?(SystemExit)
+    puts exception
+    puts exception.backtrace
+  end
 end
 
 exit run(ARGV)