acme/internal/acmeprobe: fix dropped error variable

This fixes a dropped err in runPreauthz().

Change-Id: I46f98d0a21e037a49e4b779895214538d1162376
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/206237
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

acme/internal/acmeprobe/prober.go

@@ -213,6 +213,9 @@ func (p *prober) runOrder(ctx context.Context, identifiers []acme.AuthzID) {
 
 func (p *prober) runPreauthz(ctx context.Context, identifiers []acme.AuthzID) {
 	dir, err := p.client.Discover(ctx)
+	if err != nil {
+		log.Fatalf("Discover: %v", err)
+	}
 	if dir.AuthzURL == "" {
 		log.Fatal("CA does not support pre-authorization")
 	}