add method for reading APIClient initialization errors

Description

Background

We use DockerCli as a library to create an APIClient. As far as I know, this is the only way to get an APIClient that's DOCKER_CONTEXT aware.

The Problem

If the APIClient fails to initialize, the library mysteriously exits.

https://github.com/docker/cli/blob/d1c04fb0f8db4a3c163f310652d19c65f6e2ff11/cli/command/cli.go#L106

func (cli *DockerCli) Client() client.APIClient {
	if err := cli.initialize(); err != nil {
		_, _ = fmt.Fprintf(cli.Err(), "Failed to initialize: %s\n", err)
		os.Exit(1)
	}
	return cli.client
}

This is a sad way to do error-handling:

• It's usually considered bad form for libraries to call os.Exit
• Err is a generic Writer, and you can't guarantee that the message will get flushed before the process terminates

Proposal

• Mark Client() as deprecated
• Add two new functions: ClientOrDie() and ClientOrError()
• ClientOrDie has the same signature and behavior as ClientOrDie() today
• ClientOrError returns the error to the caller

Start at cli/command/cli.go, especially DockerCli.Client and the linked initialization path, to understand the current failure behavior. Define the new ClientOrDie and ClientOrError entry points and deprecate Client; done means callers can receive initialization errors instead of the library unconditionally terminating.