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>
549d39a89f