Give what() to PanicError so that uncaught panics give proper message on
std::terminate_handler crash instead of printing just "std::exception",
for example

    terminate called after throwing an instance of 'golang::PanicError'
      what():  chan: decref: free: recvq not empty

instead of

    terminate called after throwing an instance of 'golang::PanicError'
      what():  std::exception