message: Added class and namespace to error message
diff: @@ -124,12 +124,13 @@ public:
  {}
  const std::string& operator*() const {
  + if (this->m_ptr == nullptr) throw std::runtime_error("[oatpp::data::mapping::type::String] Error: m_ptr points to null.");
  return this->m_ptr.operator*();
  }
  operator std::string() const
  {
  - if (this->m_ptr == nullptr) throw std::runtime_error("m_ptr points to null.");
  + if (this->m_ptr == nullptr) throw std::runtime_error("[oatpp::data::mapping::type::String] Error: m_ptr points to null.");
  return this->m_ptr.operator*();
  }
  