[errors] Greatly simplify common/errors package.

milo/build_source/raw_presentation/html.go

Index: milo/build_source/raw_presentation/html.go
diff --git a/milo/build_source/raw_presentation/html.go b/milo/build_source/raw_presentation/html.go
index 2021a2846c25b78830c198e509efa9467f7f24cb..a6bc384b4503ee2c740be8e36e33e17f30c371e4 100644
--- a/milo/build_source/raw_presentation/html.go
+++ b/milo/build_source/raw_presentation/html.go
@@ -87,9 +87,7 @@ func resolveHost(host string) (string, error) {
 	case defaultLogDogHost, "luci-logdog-dev.appspot.com":
 		return host, nil
 	default:
-		return "", errors.Reason("host %(host)q is not whitelisted").
-			D("host", host).
-			Err()
+		return "", errors.Reason("host %q is not whitelisted", host).Err()
 	}
 }