fix 'missing }' error message

lib/less/parser.js

@@ -309,11 +309,12 @@ less.Parser = function Parser(env) {
                     }
                 }
                 if (level > 0) {
-                    throw {
-                        type: 'Syntax',
-                        message: "Missing closing `}`",
+                    return callback(new(LessError)({
+                        index: i,
+                        type: 'Parse',
+                        message: "missing closing `}`",
                         filename: env.filename
-                    };
+                    }, env));
                 }
 
                 return chunks.map(function (c) { return c.join('') });;