Reduced clutter in monadic code

+ Eliminated vairous ad-hoc coersion functions between specific monads 
  (IO, Err, IOE, Check) in favor of more general lifting functions
  (liftIO, liftErr).
+ Generalized many basic monadic operations from specific monads to
  arbitrary monads in the appropriate class (MonadIO and/or ErrorMonad),
  thereby completely eliminating the need for lifting functions in lots
  of places.

This can be considered a small step forward towards a cleaner
compiler API and more malleable compiler code in general.

src/compiler/GF.hs

@@ -12,7 +12,6 @@ import Data.Version
 import System.Directory
 import System.Environment (getArgs)
 import System.Exit
-import System.IO
 import GF.System.Console (setConsoleEncoding)
 
 main :: IO ()
@@ -23,8 +22,8 @@ main = do
     Ok (opts,files) -> do curr_dir <- getCurrentDirectory
                           lib_dir  <- getLibraryDirectory opts
                           mainOpts (fixRelativeLibPaths curr_dir lib_dir opts) files
-    Bad err         -> do hPutStrLn stderr err
-                          hPutStrLn stderr "You may want to try --help."
+    Bad err         -> do ePutStrLn err
+                          ePutStrLn "You may want to try --help."
                           exitFailure
 
 mainOpts :: Options -> [FilePath] -> IO ()