Give readProcess' more sensible error messages.

Also a bunch of refactoring for GhcModError

Language/Haskell/GhcMod/Error.hs

@@ -0,0 +1,39 @@
+{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
+module Language.Haskell.GhcMod.Error (
+    GhcModError(..)
+  , modifyError
+  , modifyError'
+  , tryFix
+  , module Control.Monad.Error
+  , module Exception
+  ) where
+
+import Control.Monad.Error (MonadError(..), Error(..))
+import Exception
+
+data GhcModError = GMENoMsg
+                 -- ^ Unknown error
+                 | GMEString String
+                 -- ^ Some Error with a message. These are produced mostly by
+                 -- 'fail' calls on GhcModT.
+                 | GMECabalConfigure GhcModError
+                 -- ^ Configuring a cabal project failed.
+                 | GMEProcess [String] GhcModError
+                 -- ^ Launching an operating system process failed. The first
+                 -- field is the command.
+                   deriving (Eq,Show)
+
+instance Error GhcModError where
+    noMsg = GMENoMsg
+    strMsg = GMEString
+
+modifyError :: MonadError e m => (e -> e) -> m a -> m a
+modifyError f action = action `catchError` \e -> throwError $ f e
+
+infixr 0 `modifyError'`
+modifyError' :: MonadError e m => m a -> (e -> e) -> m a
+modifyError' = flip modifyError
+
+tryFix :: MonadError e m => m a -> (e -> m ()) -> m a
+tryFix action fix = do
+  action `catchError` \e -> fix e >> action