Quick fix to address the borken build caused by #15791

This is pretty lame though, so feel free to send a PR with a better fix! ;)

cc @zev @tenderlove

activerecord/lib/active_record/core.rb

@@ -181,7 +181,7 @@ def find_by(*args)
       end
 
       def find_by!(*args)
-        find_by(*args) or raise RecordNotFound
+        find_by(*args) or raise RecordNotFound.new("Couldn't find #{name}")
       end
 
       def initialize_generated_modules