storage: fix flaky HMAC key test
storage: fix flaky HMAC key test

Make sure the expected error here is correct.

Fixes #1792

Change-Id: Ib672ee5d50d80d32e27b45becd2471a99db7f62f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/52290
Reviewed-by: Frank Natividad <52e924866916bede4f49dda05c657c45c4ff3b20@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go