spanner: fix TestTransaction_SessionNotFound test
spanner: fix TestTransaction_SessionNotFound test

This is caused by an upgrading to protobuf 1.4. So

{*spanner.Error}.err.(*status.statusError).Details

has been changed to:

{*spanner.Error}.err.(*status.Error).Details.

Fixes #1949.

Change-Id: I1e4cad4fb806d4265f304b0c2115c12e1537624e
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/55237
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Knut Olav Løite <62634d14bf6dff6edb4b7af974862bb227095f71@gmail.com>

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