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...
Merge branch '1.2-dev' of github.com:TRIQS/triqs into 1.2-dev