win32/HResult: add MakeHResultError()

None of the current FormatHResultError() callers need the format string.

src/win32/Com.hxx

@@ -31,7 +31,7 @@ public:
 	COM() {
 		if (HRESULT result = CoInitializeEx(nullptr, COINIT_MULTITHREADED);
 		    FAILED(result)) {
-			throw FormatHResultError(
+			throw MakeHResultError(
 				result,
 				"Unable to initialize COM with COINIT_MULTITHREADED");
 		}
@@ -39,7 +39,7 @@ public:
 	COM(bool) {
 		if (HRESULT result = CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
 		    FAILED(result)) {
-			throw FormatHResultError(
+			throw MakeHResultError(
 				result,
 				"Unable to initialize COM with COINIT_APARTMENTTHREADED");
 		}

src/win32/ComPtr.hxx

@@ -85,7 +85,7 @@ public:
 			::CoCreateInstance(class_id, unknown_outer, class_context,
 					   __uuidof(T), reinterpret_cast<void **>(&ptr));
 		if (FAILED(result)) {
-			throw FormatHResultError(result, "Unable to create instance");
+			throw MakeHResultError(result, "Unable to create instance");
 		}
 	}
 

src/win32/HResult.hxx

@@ -74,6 +74,13 @@ static inline const std::error_category &hresult_category() noexcept {
 	return hresult_category_instance;
 }
 
+inline std::system_error
+MakeHResultError(HRESULT result, const char *msg) noexcept
+{
+	return std::system_error(std::error_code(result, hresult_category()),
+				 msg);
+}
+
 gcc_printf(2, 3) std::system_error
 FormatHResultError(HRESULT result, const char *fmt, ...) noexcept;