util/vizerror: add new package for visible errors

Signed-off-by: Will Norris <will@tailscale.com>

util/vizerror/vizerror_test.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package vizerror
+
+import (
+	"errors"
+	"io/fs"
+	"testing"
+)
+
+func TestNew(t *testing.T) {
+	err := New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func TestErrorf(t *testing.T) {
+	err := Errorf("%w", fs.ErrNotExist)
+
+	if got, want := err.Error(), "file does not exist"; got != want {
+		t.Errorf("Errorf().Error() = %q, want %q", got, want)
+	}
+
+	// ensure error wrapping still works
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("error chain does not contain fs.ErrNotExist")
+	}
+}