Improve local dev override docs and add read-only example

internal/provider/diagnostics.go

@@ -0,0 +1,23 @@
+package provider
+
+import (
+	"errors"
+
+	"github.com/dynu/terraform-provider-dynu/internal/dynuclient"
+)
+
+func diagnosticSummary(defaultSummary string, err error) string {
+	var apiErr *dynuclient.APIError
+	if !errors.As(err, &apiErr) {
+		return defaultSummary
+	}
+
+	switch apiErr.StatusCode {
+	case 401, 403:
+		return defaultSummary + " (authentication failed)"
+	case 404:
+		return defaultSummary + " (not found)"
+	default:
+		return defaultSummary
+	}
+}