IMPLEMENT_ME should signal an error, not crash

[Bike]

It's defined in gctools/hardErrors.h, and I know nothing about hard errors.

Upon hitting one clasp stops itself and you get a message like

clasp (../../src/main/main.cc:362): The generic catch(...) caught an unhandled exception of type 0x113fb3920 - this is a bug. Please report this to clasp support !
*** BACKTRACE:

and a uselessly short backtrace, after the implement_me message.