storage: fix reader test
storage: fix reader test

Give the error the right type.

Change-Id: Ieca39a047790cb5c3d6aa2aff91987594598e134
Reviewed-on: https://code-review.googlesource.com/26431
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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