chore(JSON): add stringify unicode tests #4707 (#4720)

* chore(JSON): add stringify unicode tests #4707

* enh(Mutex): Error code for pthread_mutex_lock failure #4712

Foundation/include/Poco/Error.h

@@ -43,6 +43,9 @@ public:
 	static std::string getMessage(int errorCode);
 		/// Utility function translating numeric error code to string.
 #endif
+
+	static std::string getLastMessage();
+		/// Utility function returning the last error message.
 };