message: elle.exception: a means to force the display of the backtrace
diff: #include <elle/Exception.hh>
  #include <elle/assert.hh>
  +#include <elle/os/environ.hh>
  #include <elle/serialization/Serializer.hh>
  namespace elle
  @@ -54,6 +55,8 @@ namespace elle
  Exception const& e)
  {
  s << e.what();
  + if (os::getenv("ELLE_DEBUG_BACKTRACE", false))
  + s << e.backtrace();
  return s;
  }
  