add domain_error enum with std::error_code support

I want to us std::expected<T, std::error_code> for error handling, so
proper support of std::error_code in terms of the fix bounded context is
needed

src/domain/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(DOMAIN_SOURCES
     application_service.cpp
+    domain_error.cpp
     title.cpp
     )
 

src/domain/domain_error.cpp

@@ -0,0 +1,35 @@
+#include "domain_error.hpp"
+
+namespace fix::domain {
+namespace {
+
+std::string error_message(fix::domain::domain_error error) {
+  switch (error) {
+  case domain_error::MISSING_IMPLEMENTATION:
+    return "feature is not implemented yet";
+  }
+  return "unknown domain error";
+}
+
+// =========================================================
+// wiring to enable use of std::error_code with domain_error
+// =========================================================
+
+struct fix_error_category_t : std::error_category {
+  [[nodiscard]] char const* name() const noexcept override {
+    return "fix_domain_error";
+  }
+
+  [[nodiscard]] std::string message(int condition) const override {
+    return error_message(static_cast<fix::domain::domain_error>(condition));
+  };
+};
+
+const fix_error_category_t fix_error_category;
+} // namespace
+
+std::error_code make_error_code(domain_error error) {
+  return {static_cast<int>(error), fix_error_category};
+}
+
+} // namespace fix::domain

src/domain/domain_error.hpp

@@ -0,0 +1,26 @@
+#ifndef FIX_SRC_DOMAIN_DOMAIN_ERROR_HPP
+#define FIX_SRC_DOMAIN_DOMAIN_ERROR_HPP
+
+#include <system_error>
+
+namespace fix::domain {
+
+enum class domain_error {
+  MISSING_IMPLEMENTATION = 1,
+};
+
+}
+
+// =========================================================
+// wiring to enable use of std::error_code with domain_error
+// =========================================================
+namespace std {
+template<>
+struct is_error_code_enum<fix::domain::domain_error> : true_type {};
+} // namespace std
+
+namespace fix::domain {
+std::error_code make_error_code(domain_error);
+}
+
+#endif // FIX_SRC_DOMAIN_DOMAIN_ERROR_HPP

test/domain/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(DOMAIN_TEST_SOURCES
     application_service_test.cpp
+    domain_error_test.cpp
     title_test.cpp
     )
 

test/domain/domain_error_test.cpp

@@ -0,0 +1,11 @@
+#include <catch2/catch.hpp>
+
+#include "domain_error.hpp"
+
+using namespace fix::domain;
+
+TEST_CASE("Domain errors can be used with std::error_code") {
+  std::error_code error_code = domain_error::MISSING_IMPLEMENTATION;
+
+  CHECK(error_code.message() == "feature is not implemented yet");
+}