Short-circuit retries on context errors. (#151)
Short-circuit retries on context errors. (#151)

Without this, cancellation leads to weird log messages, like
Request failed, backing-off for 1s: context cancelled. It also means the
function doesn't return as quickly as it could under cancellation.
RJPercival/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go