Add identifier & error message constructor to ValidationError (#209)

Author: gordysc

src/Ardalis.Result/ValidationError.cs

@@ -8,6 +8,12 @@ public ValidationError()
 
         public ValidationError(string errorMessage) => ErrorMessage = errorMessage;
 
+        public ValidationError(string identifier, string errorMessage)
+        {
+            Identifier = identifier;
+            ErrorMessage = errorMessage;
+        }
+
         public ValidationError(string identifier, string errorMessage, string errorCode, ValidationSeverity severity)
         {
             Identifier = identifier;