Re-use the caught exception's error message in the parse error call.

lib/parse.js

@@ -484,7 +484,7 @@ function tokenizer($TEXT, filename, html5_comments) {
         try {
           r = new RegExp(regexp, mods);
         } catch(e) {
-          parse_error("Invalid regular expression");
+          parse_error(e.message);
         }
         return token("regexp", r);
     });