[Bug libstdc++/107792] Output of default contract violation handler could be improved

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107792

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> default std::handle_contract_violation called: 

N.B. The function is not in namespace std.

This seems better:

default handle_contract_violation called [default:default:NEVER]
in function f at fail.cc:1: r > 0
terminate called without an active exception
Aborted (core dumped)


We could say "[level:default role:default continue:NEVER]" instead if the
shorter form is too obscure.

Maybe instead of saying the default handler was called, we should just say a
contract was violated. That's what users actually care about, not the handler
itself:

contract violation in function f at fail.cc:1: r > 0
[level:default, role:default, continuation mode:never]
terminate called without an active exception
Aborted (core dumped)