cli-plugins/manager: fix Plugin marshaling with regular errors

Go does not by default marshal `error` type fields to JSON. The manager
package therefore implemented a `pluginError` type that implements
[encoding.TextMarshaler]. However, the field was marked as a regular
`error`, which made it brittle; assining any other type of error would
result in the error being discarded in the marshaled JSON (as used in
`docker info` output), resulting in the error being marshaled as `{}`.

This patch adds a custom `MarshalJSON()` on the `Plugin` type itself
so that any error is rendered. It checks if the error used already
implements [encoding.TextMarshaler], otherwise wraps the error in
a `pluginError`.

[encoding.TextMarshaler]: https://pkg.go.dev/encoding#TextMarshaler

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/info_test.go

@@ -2,6 +2,7 @@ package system
 
 import (
 	"encoding/base64"
+	"errors"
 	"net"
 	"testing"
 	"time"
@@ -226,7 +227,7 @@ var samplePluginsInfo = []pluginmanager.Plugin{
 	{
 		Name: "badplugin",
 		Path: "/path/to/docker-badplugin",
-		Err:  pluginmanager.NewPluginError("something wrong"),
+		Err:  errors.New("something wrong"),
 	},
 }