abstract exception handler/logger into its own file

test/regression.sml

@@ -9,8 +9,8 @@ struct
     else
       let
         val chr = String.sub (str, pos)
+        val () = ExceptionLogger.addCommand (InputMsg.CHAR_EVENT chr)
         val app = AppUpdate.update (app, InputMsg.CHAR_EVENT chr)
-                  handle _ => raise Fail (Int.toString pos)
       in
         updateLoop (pos + 1, str, app)
       end

test/test.sml

@@ -10,6 +10,7 @@ struct
       val tests = concat tests
     in
       runWithConfig [Configuration.PrintPassed false] tests
+      handle e => ExceptionLogger.log e
     end
 end