pkg/httphelper: Make ServiceUnavailableError retryable
pkg/httphelper: Make ServiceUnavailableError retryable

This is particularly beneficial to the scheduler so it retries job
persistence duriing a controller deployment (which has been a source of
missed events in CI).

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

philiplb/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn,lmars/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,flynn/flynn,flynn/flynn,philiplb/flynn