trillian/hammer: use glog.Exitf()
trillian/hammer: use glog.Exitf()

This ensures the log is flushed and gives a definitive
reason for exit.

RJPercival/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go