Quick fix to render some parser error messages from UTF-8-encoded source files correctly.

The parser works on raw byte sequences read from source files. If parsing
succeeds the raw byte sequences are converted to proper Unicode characters 
in a later phase. But the parser calls the function buildAnyTree, which can 
fail and generate error messages containing source code fragments, which might
then containing raw byte sequences. To render these error messages correctly, 
they need to be converted in accordance with the coding flag in the source 
file. This is now done for UTF-8-encoded source files, but should ideally also
be done for other character encodings. (Latin-1-encoded files never suffered 
from this problem, since raw bytes are proper Unicode characters in this case.)

src/compiler/GF/Grammar/Parser.y

@@ -17,6 +17,8 @@ import GF.Grammar.Macros
 import GF.Grammar.Lexer
 import qualified Data.ByteString.Char8 as BS
 import GF.Compile.Update (buildAnyTree)
+import Codec.Binary.UTF8.String(decodeString)
+import Data.Char(toLower)
 }
 
 %name pModDef ModDef
@@ -116,7 +118,7 @@ ModDef
                                       jments <- mapM (checkInfoType mtype) jments
                                       defs <- case buildAnyTree id jments of
                                                 Ok x    -> return x
-                                                Bad msg -> fail msg
+                                                Bad msg -> fail (optDecode opts msg)
                                       return (id, ModInfo mtype mstat opts extends with opens [] "" Nothing defs)  }
 
 ModHeader :: { SourceModule }
@@ -605,6 +607,12 @@ Posn
 happyError :: P a
 happyError = fail "syntax error"
 
+-- Quick fix to render error messages from UTF-8-encoded source files correctly.
+optDecode opts =
+    if map toLower (flag optEncoding opts) `elem` ["utf8","utf-8"]
+    then decodeString
+    else id
+
 mkListId,mkConsId,mkBaseId  :: Ident -> Ident
 mkListId = prefixId (BS.pack "List")
 mkConsId = prefixId (BS.pack "Cons")