Just a small side note, you don't even need to write your errorCode function. That and a number of other useful functions are provided in the errors package (http://hackage.haskell.org/package/errors). Also, I like to use string error messages because they're easier to grep for. Here's what your response function would look like with those changes.
response :: Messy -> Either String String
response message = do
b <- note "error in body" $ body message
lines <- note "error in text" $ text b
note "error in atMay" $ lines `atMay` 10
For anyone reading this, I want to confirm that mightybyte's code would be far more idiomatic. I would never use integer error codes in a Haskell program, but I wanted to make the clear connection to C idioms. Also, using the errors package would be more appropriate than rolling your own function there, but I wanted to emphasize that it was simple, two line function, and not any fancy wizardry that requires caring about category theory.
EDIT: rechecking the errors library, the final line would probably be more clear with
