Better error when compiling level definitions

[omelkonian]

A user is allowed to compile a level definition, resulting in confusing error messages:

open import Agda.Primitive using (Level; lzero)

zeroLvl : Level
zeroLvl = lzero
{-# COMPILE AGDA2HS zeroLvl #-}

Functions defined with absurd patterns exclusively are not supported. Use function `error` from the Haskell.Prelude instead.

We should give a more appropriate level (e.g. Cannot compile level definitions), or at least a warning for the user to not use compile pragmas on level definitions.

[omelkonian]

This was fixed by PR #358, which now more adequately reports:

Bad Haskell type: Level