PyUnicode_EncodeFS() raises an exception if _Py_wchar2char() fails

 * Add error_pos optional argument to _Py_wchar2char()
 * PyUnicode_EncodeFS() raises a UnicodeEncodeError or MemoryError if
   _Py_wchar2char() fails
diff --git a/Include/fileutils.h b/Include/fileutils.h
index 11ebebf..2fade9b 100644
--- a/Include/fileutils.h
+++ b/Include/fileutils.h
@@ -10,7 +10,8 @@
     size_t *size);
 
 PyAPI_FUNC(char*) _Py_wchar2char(
-    const wchar_t *text);
+    const wchar_t *text,
+    size_t *error_pos);
 
 #if defined(HAVE_STAT) && !defined(MS_WINDOWS)
 PyAPI_FUNC(int) _Py_wstat(