Give a good error message if an invalid regular expression is found.

lib/parse.js

@@ -480,7 +480,13 @@ function tokenizer($TEXT, filename, html5_comments) {
             regexp += ch;
         }
         var mods = read_name();
-        return token("regexp", new RegExp(regexp, mods));
+        var r;
+        try {
+          r = new RegExp(regexp, mods);
+        } catch(e) {
+          parse_error("Invalid regular expression");
+        }
+        return token("regexp", r);
     });
 
     function read_operator(prefix) {