The big difference here is that the OCaml compiler has a lot less work to do. It's not that the Haskell error messages are inadequate (they are actually pretty good), but that the amount of compiler features and type gymnastics make the errors deeper and more complex. For example, if you get the parens wrong in a >> or >>=, you'll get some rather cryptic error that only hits home once you've seen it a few times, as opposed to "did you mean to put parens over there?"
The big difference here is that the OCaml compiler has a lot less work to do. It's not that the Haskell error messages are inadequate (they are actually pretty good), but that the amount of compiler features and type gymnastics make the errors deeper and more complex. For example, if you get the parens wrong in a >> or >>=, you'll get some rather cryptic error that only hits home once you've seen it a few times, as opposed to "did you mean to put parens over there?"