retryingClient.get checked its context right after each request and
returned that request's error, so a deadline that passed while a 503 was
on its way came back as the 503. TestGetStopsWaitingWhenCanceled failed
on that now and then on a loaded host. When the context ends before a
retry, during the request or during the wait, get now returns an error
wrapping both the context's error and the last try's error.
A new test covers the context ending during the request, and
TestGetStopsWaitingWhenCanceled now covers it ending during the wait.
Both answer through a transport rather than a server, so neither depends
on how fast the 503 arrives.
Model: opus-5-5
sneak