buildlet: copy GCE errors
buildlet: copy GCE errors

This was intended to be part of CL 426794.

Updates golang/go#48857

Change-Id: Ic317c7fb633910c969f27bd220e17dba148e8909
Reviewed-on: https://go-review.googlesource.com/c/build/+/426798
Run-TryBot: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build