LTA error message with invalid backend

[coke]

$ perl Configure.pl --backends=nope
===ATTENTION===
  No --prefix supplied, building and installing to /Volumes/BUKKIT/Users/coke/sandbox/nqp/install
Unknown backend: 'nope'; Known backends: moar, jvm, js
 at /Volumes/BUKKIT/Users/coke/sandbox/nqp/3rdparty/nqp-configure/lib/NQP/Config.pm line 34.
	NQP::Config::__ANON__("Unknown backend: 'nope'; Known backends: moar, jvm, js\x{a}") called at /Volumes/BUKKIT/Users/coke/sandbox/nqp/tools/lib/NQP/Config/NQP.pm line 21
	NQP::Config::NQP::configure_backends(NQP::Config::NQP=HASH(0x7ffcd0020738)) called at Configure.pl line 82

Do we need the stack trace?

[vrurg]

I'd rather have them there as at some point they might help in debugging based on a user report. And since it's a building process, we're not restricted by usability guidelines.