proxy: Fix CI fail from format token in Error call
proxy: Fix CI fail from format token in Error call

go vet caused a build fail in https://travis-ci.org/mholt/caddy/jobs/248392875:
   upstream_test.go:480::error: possible formatting directive in Error call (vet)

This patch changes the Error call added in commit 078c991574647c27 to
an Errorf call to support the use of the %d token.

joshix/caddy,devangels/caddy,dusty/caddy,yukimochi/caddy,dusty/caddy,captncraig/caddy,yukimochi/caddy,joshix/caddy,tobya/caddy,joshix/caddy,cez81/caddy,tobya/caddy,cez81/caddy,dusty/caddy,tobya/caddy,devangels/caddy,devangels/caddy,dusty/caddy,tobya/caddy,captncraig/caddy,yukimochi/caddy,cez81/caddy,captncraig/caddy