sml: fix mlton self-host. Full syntax err in catch*

In mlton mode, syntax errors caught in the try/catch block were being
processed as "SyntaxError" without any additional context/message.
Explicitly pass the full message always.

impls/sml/step9_try.sml

@@ -71,6 +71,7 @@ and evalTry e [a, LIST ([SYMBOL "catch*", b, c],_)] = (eval e a handle ex => eva
 and evalCatch e b ex body = eval (bind [b, exnVal ex] e) body
 
 and exnVal (MalException x)    = x
+  | exnVal (SyntaxError msg)   = STRING msg
   | exnVal (NotDefined msg)    = STRING msg
   | exnVal (NotApplicable msg) = STRING msg
   | exnVal (OutOfBounds msg)   = STRING msg

impls/sml/stepA_mal.sml

@@ -71,6 +71,7 @@ and evalTry e [a, LIST ([SYMBOL "catch*", b, c],_)] = (eval e a handle ex => eva
 and evalCatch e b ex body = eval (bind [b, exnVal ex] e) body
 
 and exnVal (MalException x)    = x
+  | exnVal (SyntaxError msg)   = STRING msg
   | exnVal (NotDefined msg)    = STRING msg
   | exnVal (NotApplicable msg) = STRING msg
   | exnVal (OutOfBounds msg)   = STRING msg

impls/tests/step9_try.mal

@@ -41,6 +41,7 @@
 (try* (map throw (list "my err")) (catch* exc exc))
 ;=>"my err"
 
+
 ;;
 ;; Testing builtin functions