This piggybacks on googleapis#4797 to allow storage to expose wrapped
service errors when a call retries without success until timeout
or cancellation.

I also updated all checks for context sentinel errors in storage
to use xerrors.Is instead of strict equality. Users of this
package should do the same. I'll update the documentation on
errors from this package in a subsequent PR.

We will have to bump the dependency on the root module before
merging this change I believe.

Fixes googleapis#4197