fixed exception constructor for msvc

libdevcore/Exceptions.h

@@ -33,7 +33,7 @@ namespace dev
 // base class for all exceptions
 struct Exception: virtual std::exception, virtual boost::exception
 {
-	Exception(std::string _message = {}): m_message(std::move(_message)) {}
+	Exception(std::string _message = std::string()): m_message(std::move(_message)) {}
 	const char* what() const noexcept override { return m_message.empty() ? std::exception::what() : m_message.c_str(); }
 
 private: