Prevent Unicode characters in warning messages from stopping grammar compilation

When compiling a grammar containing characters that are not supported in the
current locale, warning messages could cause GF fail with 

  hPutChar: invalid argument (Invalid or incomplete multibyte or wide character)

With this quick fix, warning messages that can not be displayed are silently
truncated instead, and compilation continues.

src/compiler/GF/Compile.hs

@@ -83,10 +83,12 @@ intermOut opts d doc
 
 warnOut opts warnings
   | null warnings = return ()
-  | otherwise     = ioeIO (hPutStrLn stderr $ 
+  | otherwise     = ioeIO $ hPutStrLn stderr ws `catch` oops
-                                     if flag optVerbosity opts == Normal
+  where
-                                       then ('\n':warnings)
+    oops _ = hPutStrLn stderr "" -- prevent crash on character encoding problem
-                                       else warnings)
+    ws = if flag optVerbosity opts == Normal
+         then '\n':warnings
+         else warnings
 
 -- | the environment
 type CompileEnv = (Int,SourceGrammar,ModEnv)