New: Improve validation messages

(cherry picked from commit a117001de673e80abd90d54a34a7c86292b3a649)

src/NzbDrone.Core/Validation/UrlValidator.cs

@@ -14,7 +14,7 @@ namespace NzbDrone.Core.Validation
 
     public class UrlValidator : PropertyValidator
     {
-        protected override string GetDefaultMessageTemplate() => "Invalid Url";
+        protected override string GetDefaultMessageTemplate() => "Invalid Url: '{url}'";
 
         protected override bool IsValid(PropertyValidatorContext context)
         {
@@ -23,6 +23,8 @@ namespace NzbDrone.Core.Validation
                 return false;
             }
 
+            context.MessageFormatter.AppendArgument("url", context.PropertyValue.ToString());
+
             return context.PropertyValue.ToString().IsValidUrl();
         }
     }