image: add missing error check in test
image: add missing error check in test

Change-Id: Ia42a4a658e4207cc1f036f2faeac011e71edad77
GitHub-Last-Rev: b384f81799fcd404f53647d8a3015487ea240c6f
GitHub-Pull-Request: golang/go#30012
Reviewed-on: https://go-review.googlesource.com/c/go/+/160436
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,golang/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,akutz/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go