std::future_error::what
From cppreference.com
                    
                                        
                    < cpp | thread | future error
                    
                                                            
                    |   virtual const char* what() const noexcept;  | 
(since C++11) | |
Returns the explanatory string.
Parameters
(none)
Return value
Pointer to a null-terminated string with explanatory information.
See also
|    returns the error code  (public member function)  |