Fixing access of boost::diagnostic_information() for Exceptions

- Figured it out with help from @arkpar. Thanks!

libdevcore/Exceptions.h

@@ -33,8 +33,8 @@ namespace dev
 // base class for all exceptions
 struct Exception: virtual std::exception, virtual boost::exception
 {
-	Exception(std::string _message = {}) : m_message(std::move(_message)) {}
-	const char* what() const noexcept override { return m_message.c_str(); }
+	Exception(std::string _message = {}): m_message(std::move(_message)) {}
+	const char* what() const noexcept override { return m_message.empty() ? std::exception::what() : m_message.c_str(); }
 
 private:
 	std::string m_message;