Introduce MessageError namespace

MessageError.cpp

@@ -0,0 +1,51 @@
+#include "MessageError.hpp"
+
+/******************************************************************************/
+// Private Implementation
+/******************************************************************************/
+
+#pragma mark Private Implementation
+
+namespace
+{
+    const struct final : public std::error_category
+    {
+        const char *
+        name() const noexcept override
+        {
+            return "message";
+        }
+        
+        std::string
+        message(int const ev) const override
+        {
+            using MessageError::Code;
+            
+            switch (static_cast<Code>(ev))
+            {
+                case Code::json_parsing_error: return "json parsing error";
+                case Code::json_not_an_object: return "json not an object";
+            
+                default: return "message error";
+            }
+        }
+    }
+    Category;
+}
+
+/******************************************************************************/
+// Private Implementation
+/******************************************************************************/
+
+#pragma mark - Implementation
+
+namespace MessageError
+{
+    std::error_category const &
+    category() noexcept
+    {
+        return Category;
+    }
+}
+
+/******************************************************************************/