title: Error message from ModelState.AddModelError() doesn't appear
body: <p>My program has users who can create a project (which behave like folders) and inside the projects the user can add documents.
  The user can than share the project with another user.</p>
  
  <p>I have prevented that a user can add the same person twice in the same project but I can't seem to display the error message I want.</p>
  
  <p>The email has Required attributes and those error messages appear (both if it is invalid and if the box is empty).</p>
  
  <p>Am I not using the ModelState.AddErrorMessage() right?</p>
  
  <p>This is how the variable is in the Model</p>
  
  <pre><code>    [EmailAddress(ErrorMessage ="Please enter a valid email address!")]
      [Display(Name = "Email")]
      [Required(ErrorMessage = "You must enter an email!")]
      public string email { get; set; }
  </code></pre>
  
  <p>This is a part of my Controller</p>
  
  <pre><code>//Prevent double share
  List&lt;ProjectModel&gt; userProjects = projectService.getAllProjects(user.email);
      foreach(var proj in userProjects)
      {
          if(proj.name == userService.getProjectByID(urlInt).name)
          {
              //This is the error message I am trying to display
              ModelState.AddModelError(user.email, "This user already has access to this project!");
              return View(user);
          }
      }
  userService.share(user.email, urlInt);
  return RedirectToAction("../Dashboard/Dashboard");
  </code></pre>
  
  <p>This is my View</p>
  
  <pre><code>@using (Html.BeginForm("Share", "User", FormMethod.Post, new { @class = "add-document-form" }))
  { 
      &lt;div class="add-document-div"&gt;
          &lt;div class="form-group"&gt;
              &lt;label for="email" class="control-label"&gt;Email address&lt;/label&gt;
              &lt;br /&gt;
              @Html.TextBoxFor(m =&gt; m.email, new { @class = "form-conrtol" })
              &lt;br /&gt;
  
              &lt;!--Here is the validation for the email button--&gt;
              @Html.ValidationMessageFor(m =&gt; m.email, "", new { @class = "text-danger" })
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;input type="submit" class="btn btn-fab" value="Send invitation" /&gt;
  }
  </code></pre>