router/proxyproto: Don't call t.Fatalf from goroutines
router/proxyproto: Don't call t.Fatalf from goroutines

It is unsafe to call t.Fatalf from goroutines, so change to
t.Errorf, which is safe.

Closes #3708

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

philiplb/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,flynn/flynn