test(e2e): the bucket read-only upload check passes on any transport failure #5317

The read-only upload check in the verify-s3-access step of hack/e2e-chainsaw/bucket/chainsaw-test.yaml counts any non-zero mc cp exit other than 124 as the expected denial. A broken port-forward or a 5xx from the gateway therefore passes as "read-only user was denied", so an accessPolicy=readonly regression would go unnoticed whenever the transport fails.

This is the last instance of the class described in #3700. #5175 fixes the shared assert_file_lacks_pattern helper and closes #3700, but it does not touch this check.

Fix: count the attempt as denied only when mc reports the S3 AccessDenied error, and fail with a named message on any other non-zero exit.