test(render): improve test coverage

format.go

@@ -1,17 +1,22 @@
 package render
 
 import (
+	"fmt"
 	"io"
 )
 
+var ErrUnsupportedFormat = fmt.Errorf("%w: unsupported format", Err)
+
 // FormatRenderer is a renderer that delegates rendering to another renderer
 // based on a format value.
 type FormatRenderer struct {
+	// Renderers is a map of format names to renderers. When Render is called,
+	// the format is used to look up the renderer to use.
 	Renderers map[string]Renderer
 }
 
 // Render renders a value to an io.Writer using the specified format. If the
-// format is not supported, ErrCannotRender is returned.
+// format is not supported, ErrUnsupportedFormat is returned.
 //
 // If the format is supported, but the value cannot be rendered to the format,
 // the error returned by the renderer is returned. In most cases this will be
@@ -23,5 +28,5 @@ func (r *FormatRenderer) Render(w io.Writer, format string, v any) error {
 		return renderer.Render(w, v)
 	}
 
-	return ErrCannotRender
+	return ErrUnsupportedFormat
 }