added user not found page

src/BirdsiteLive/Controllers/UsersController.cs

@@ -36,21 +36,31 @@ namespace BirdsiteLive.Controllers
         }
         #endregion
 
+        [Route("/users")]
+        public IActionResult Index()
+        {
+            var r = Request.Headers["Accept"].First();
+            if (r.Contains("application/activity+json")) return NotFound();
+            return View("UserNotFound");
+        }
+
         [Route("/@{id}")]
         [Route("/users/{id}")]
         public IActionResult Index(string id)
         {
             var user = _twitterService.GetUser(id);
-            if (user == null) return NotFound();
 
             var r = Request.Headers["Accept"].First();
             if (r.Contains("application/activity+json"))
             {
+                if (user == null) return NotFound();
                 var apUser = _userService.GetUser(user);
                 var jsonApUser = JsonConvert.SerializeObject(apUser);
                 return Content(jsonApUser, "application/activity+json; charset=utf-8");
             }
 
+            if (user == null) return View("UserNotFound");
+
             var displayableUser = new DisplayTwitterUser
             {
                 Name = user.Name,

src/BirdsiteLive/Views/Users/UserNotFound.cshtml

@@ -0,0 +1,11 @@
+@{
+    ViewData["Title"] = "User";
+}
+
+<div class="col-12 col-sm-10 col-md-8 col-lg-6 mx-auto text-center">
+    <h3>User not found</h3>
+    
+    <a href="/" class="btn btn-primary">
+        Back
+    </a>
+</div>