Set errmsg to exterr instead of err which will always be zero.

src/Fl_Native_File_Chooser_WIN32.cxx

@@ -502,7 +502,7 @@ int Fl_WinAPI_Native_File_Chooser_Driver::showfile() {
     if ( exterr == 0 ) return(1);       // user hit cancel
     // Otherwise, an error occurred..
     char msg[80];
-    sprintf(msg, "CommDlgExtendedError() code=%d", err);
+    sprintf(msg, "CommDlgExtendedError() code=%d", exterr);
     errmsg(msg);
     return(-1);
   }