net/http/httputil: t.Error -> t.Errorf
net/http/httputil: t.Error -> t.Errorf

Found by vet.

Change-Id: I09b79d68c7a5fc97e0edda4700a82bfbb00a4f45
Reviewed-on: https://go-review.googlesource.com/28486
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Hudson-Doyle <5e87ffb38a52af6fb9a19b679e9122c7a4d601a5@canonical.com>

golang/go,linux-on-ibm-z/go,polyverse-security/go,goodlang/good,golang/go,codestation/go,ImJasonH/go,josharian/go,derekmarcotte/go,suncycheng/go,momchil-velikov/go,vsekhar/elastic-go,Cofyc/go,josharian/go,momchil-velikov/go,ImJasonH/go,ImJasonH/go,josharian/go,christopher-henderson/Go,akutz/go,golang/go,cloudflare/go,goodlang/good,akutz/go,josharian/go,goodlang/good,codestation/go,CAFxX/go,Cofyc/go,suncycheng/go,polyverse-security/go,ImJasonH/go,CAFxX/go,nawawi/go,suncycheng/go,josharian/go,linux-on-ibm-z/go,josharian/go,cloudflare/go,suncycheng/go,polyverse-security/go,CAFxX/go,momchil-velikov/go,codestation/go,golang/go,codestation/go,Cofyc/go,codestation/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,linux-on-ibm-z/go,momchil-velikov/go,momchil-velikov/go,Cofyc/go,nawawi/go,akutz/go,polyverse-security/go,cloudflare/go,ImJasonH/go,suncycheng/go,suncycheng/go,cloudflare/go,vsekhar/elastic-go,momchil-velikov/go,suncycheng/go,suncycheng/go,akutz/go,goodlang/good,ImJasonH/go,momchil-velikov/go,goodlang/good,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,josharian/go,linux-on-ibm-z/go,Cofyc/go,vsekhar/elastic-go,golang/go,ImJasonH/go,derekmarcotte/go,goodlang/good,cloudflare/go,derekmarcotte/go,polyverse-security/go,Cofyc/go,CAFxX/go,derekmarcotte/go,goodlang/good,polyverse-security/go,golang/go,derekmarcotte/go,cloudflare/go,golang/go,derekmarcotte/go,Cofyc/go,polyverse-security/go,Cofyc/go,akutz/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,codestation/go,linux-on-ibm-z/go,golang/go,linux-on-ibm-z/go,codestation/go,codestation/go,momchil-velikov/go,CAFxX/go,goodlang/good,linux-on-ibm-z/go,cloudflare/go,derekmarcotte/go,vsekhar/elastic-go,vsekhar/elastic-go,vsekhar/elastic-go,nawawi/go,akutz/go,ImJasonH/go