acme: add missing err assignment check
acme: add missing err assignment check

Spotted it thanks to a proposal in
https://github.com/golang/go/issues/19727.

Change-Id: I389a3fc0db3cf64fba41c3ecd70a236917ea8fa3
Reviewed-on: https://go-review.googlesource.com/41698
Run-TryBot: Sam Whited <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@samwhited.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Sam Whited <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@samwhited.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,golang/crypto,golang/crypto,ScriptRock/crypto,ScriptRock/crypto