scanner: improve logging in Fetcher (#510)
scanner: improve logging in Fetcher (#510)

Prefix all log messages with the base URI of the relevant Log, so
that library users who deal with multiple Logs can disambiguate
the messages.

Also lower the severity of the warning about resetting EndIndex.
google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go