Simplify context done checks (#335)
Simplify context done checks (#335)

The [doc for Context.Err](https://pkg.go.dev/context?tab=doc#Context.Err) says
that the result is non-nil only if context is done, and only the two context
errors can be returned. So we can simplify logic that tests for done-ness.
google/exposure-notifications-server,google/exposure-notifications-server,google/exposure-notifications-server