Return a 400 on a invalid saml user

src/backend/app/Http/Controllers/AuthController.php

@@ -88,7 +88,7 @@ class AuthController extends ApiController
     /**
      * Handle SAML login
      *
-     * This API is only meant to be used by SAML after a login.
+     * This API is only meant to be used by SAML after a return from a login.
      *
      * @param AuthenticateUseCase $authenticateUseCase
      * @return mixed
@@ -102,8 +102,9 @@ class AuthController extends ApiController
         try {
             $authenticateUseCase->handleSamlLogin($presenter);
         } catch (UserCreationException $e) {
+            $this->setStatusCode(400);
             return $this->respondWithError(
-                'There was an error authenticating the user. Please contact an administrator.'
+                'Invalid SAML user given. If you believe this is in error, please contact and administrator.'
             );
         }