Improve logs for SRPM URL checks
Improve logs for SRPM URL checks

Logging 404 errors for header checks on possible signing intent related
URLs may be misleading unless followed by a success check or a build
failure. Instead, we log successful requests. If not SRPM is found for a
given package in the signing intent, we then log an error before
raising.

Signed-off-by: Athos Ribeiro <9cdfc39bfa10853bf9ad64090808d55fee7504ca@redhat.com>

fr34k8/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,DBuildService/atomic-reactor,projectatomic/atomic-reactor,projectatomic/atomic-reactor