spanner: GRPCStatus always returns a status based on specified Code
spanner: GRPCStatus always returns a status based on specified Code

This change fixes the behavior of GRPCStatus of Error that always
returns a Status based on Code field of Error.
Current implementation returns a Status that has Unknown code if the
base error is nil.

Change-Id: I832338d750396677b9e95d94613c78921493166f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/51113
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Knut Olav Løite <62634d14bf6dff6edb4b7af974862bb227095f71@gmail.com>
Reviewed-by: Hengfeng Li <b0a1bcb38cb78116cc20da5a7cacec7b885825f4@google.com>

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