Correct 0c34877b9babdd07f5a6e99ed172b967124b7e8e

- The char * was dangling ..
- add TRIQS_EXCEPTION_SHOW_CPP_TRACE
  if defined, the C++ trace will be added to the error message.
  Mainly for debug purpose only, it is disturbing for normal usage,
  since it leads people to believe the code has crashed...