Merge branch 'ab/usage-die-message' into gc/branch-recurse-submodules-fix

* ab/usage-die-message: config API: use get_error_routine(), not vreportf() usage.c + gc: add and use a die_message_errno() gc: return from cmd_gc(), don't call exit() usage.c API users: use die_message() for error() + exit 128 usage.c API users: use die_message() for "fatal :" + exit 128 usage.c: add a die_message() routine