options: gracefully report un-parsed upstream URL

upstreamURL is a nil pointer if there is an error parsing --upstream

options.go

@@ -142,15 +142,14 @@ func (o *Options) Validate() error {
 	for _, u := range o.Upstreams {
 		upstreamURL, err := url.Parse(u)
 		if err != nil {
-			msgs = append(msgs, fmt.Sprintf(
-				"error parsing upstream=%q %s",
-				upstreamURL, err))
-		}
+			msgs = append(msgs, fmt.Sprintf("error parsing upstream: %s", err))
+		} else {
 			if upstreamURL.Path == "" {
 				upstreamURL.Path = "/"
 			}
 			o.proxyURLs = append(o.proxyURLs, upstreamURL)
 		}
+	}
 
 	for _, u := range o.SkipAuthRegex {
 		CompiledRegex, err := regexp.Compile(u)