exception: Add exception_msg helper

Signed-off-by: Simon Rozman <simon@rozman.si>

UnitTests/main.cpp

@@ -39,7 +39,7 @@ int main(int, const char *[])
 		return 0;
 	}
 	catch (const std::exception& ex) {
-		std::cerr << ex.what() << " FAIL\n";
+		std::cerr << stdex::exception_msg(ex) << " FAIL\n";
 		return 1;
 	}
 }

include/stdex/exception.hpp

@@ -23,4 +23,21 @@ namespace stdex
 		///
 		user_cancelled(_In_opt_z_ const char* msg = "operation cancelled") : runtime_error(msg) {}
 	};
+
+	///
+	/// Concatenates error messages when exception is nested.
+	///
+	/// \param ex  Exception
+	///
+	/// \returns Concatenated exception messages with ": " delimiter.
+	///
+	inline std::string exception_msg(const std::exception& ex)
+	{
+		try { std::rethrow_if_nested(ex); }
+		catch (const std::exception& nested_ex) {
+			return std::string(ex.what()) + ": " + exception_msg(nested_ex);
+		}
+		catch (...) {}
+		return ex.what();
+	}
 }