Better error message for missing input type

Copy/Pasted from Formtastic :P

Author: elia

lib/active_admin/form_builder.rb

@@ -157,7 +157,7 @@ def input_class_by_trying(as)
         end
       end
     rescue NameError
-      raise Formtastic::UnknownInputError
+      raise Formtastic::UnknownInputError, "Unable to find input class for #{as}"
     end
 
     # This method calls the block it's passed (in our case, the `f.inputs` block)