Additional serror fields are not presented in error message

[IzabellaRaulin]

See package serror

There is a possibility to set serror fields to provide more details about occurring error, however, it is not presented in error message.

This happens when creating task failed and just serror.Error() is taken into account in error message - see core/task.go#L310

For example, for such returned serror:

return serror.New(
 ErrPluginIncompatibleWithScheduleType,
 map[string]interface{}{
 "schedule_type": fmt.Sprintf("%T", sch),
 "plugin_name":   plg.Name(),
 "plugin_type":   plg.TypeName(),
 },
)

I expected to have "schedule_type", "plugin_name" and "plugin_type" details in snap error message, but they are skipped.