Use go 1.13's new error functionality to create nested errors, and use the new errors.As() to unwrap them.

Fixes bad classification of tests that have recently started returning non-zero exit codes.

Change-Id: I4f356696d2a7ce1576be1c350fc14e156f63c659
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38130Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>