Fix: Appropriate message on error

src/Tree/Controllers/TreeController.cs

@@ -368,9 +368,11 @@ namespace Coscine.Api.Tree.Controllers
             var user = _authenticator.GetUser();
             var resource = _resourceModel.GetById(Guid.Parse(resourceId));
 
+            // Rights Matrix (https://git.rwth-aachen.de/coscine/docs/private/internal-wiki/-/blob/master/coscine/Definition%20of%20rights%20Matrix.md)
+            // - Resource: Change Resource (RCV, Metadatamanager)
             if (user == null || !_resourceModel.HasAccess(user, resource, UserRoles.Owner, UserRoles.Member))
             {
-                return BadRequest("User is no project member!");
+                return Unauthorized("The user is not allowed to save changes to the resource!");
             }
 
             if (resource.Archived == "1")