Introduce an explicit error value in the Term type

This makes it easier to treat run-time errors (e.g. caused by calls to
Predef.error) in a way that is more typical for a lazy functional language.

src/compiler/GF/Compile/Compute/AppPredefined.hs

@@ -107,6 +107,7 @@ appPredefined t = case t of
         (K s) | f == cIsUpper -> retb $ if (all isUpper s) then predefTrue else predefFalse
         (K s) | f == cToUpper -> retb $ K $ map toUpper s
         (K s) | f == cToLower -> retb $ K $ map toLower s
+        (K s) | f == cError   -> retb $ Error s
 
         _                    -> retb t