feat(daemon): add ErrorResponse to allow external error creation

[flotter]

#265 provides the ability for the daemon REST API to be extended by appending new handlers to the API list.

For example, the following new reboot command can be added now:

// requestReboot asks the daemon to gracefully shut down the system
// and issue a reboot.
func requestReboot(d *daemon.Daemon) daemon.Response {
        d.HandleRestart(restart.RestartSystem)
        result := deviceResult{}
        return daemon.SyncResponse(result)
}

Note that SyncResponse was already public, which allows the code above to work.

Following this patch, we can now reply with a suitable error response:

:
    switch(...) {
    case reboot:
        requestReboot(...)
    default:    
        return daemon.ErrorResponse(http.StatusBadRequest,"invalid media type %q", mediaType)
    {
:

[benhoyt]

This seems reasonable to me, especially given that SyncResponse and AsyncResponse are already exported.

[paul-rodriguez]

This is very straightforward

[niemeyer]

Some remarks below. Please let me know how it sounds.

[niemeyer]

Nice refactoring, thanks. Only a few superficial details:

[anpep]

Thanks for your work on this, I think those changes are solid!

[niemeyer]

One more detail about the StatusCode conversation above:

[benhoyt]

Sorry for the negativity here, but I really don't think breaking backwards-compatibility (for these minor adjustments) is worth it, and I also don't think we should import client from daemon. Detailed comments below.

[benhoyt]

Thanks for the changes -- I'm good with this change now!

[benhoyt]

This looks much better to me now, and a very lightweight PR.

[niemeyer]

Thanks for taking this through, Fred. Looks good.