add err.expose check for response handler to expose the err.message

lib/context.js

@@ -611,8 +611,10 @@ module.exports = {
     err.status = err.status || 500;
 
     // respond
+    var code = http.STATUS_CODES[err.status];
+    var msg = err.expose ? err.message : code;
     this.status = err.status;
-    this.res.end(http.STATUS_CODES[err.status]);
+    this.res.end(msg);
   },
 
   /**

test/application.js

@@ -243,6 +243,26 @@ describe('app.respond', function(){
       .end(function(){});
     })
 
+    describe('with an .expose property', function(){
+      it('should expose the message', function(done){
+        var app = koa();
+
+        app.use(function(next){
+          return function *(){
+            var err = new Error('sorry!');
+            err.status = 403;
+            err.expose = true;
+            throw err;
+          }
+        });
+
+        request(app.listen())
+        .get('/')
+        .expect(403, 'sorry!')
+        .end(done);
+      })
+    })
+
     describe('with a .status property', function(){
       it('should respond with .status', function(done){
         var app = koa();