Return error of command exec in SSH NativeClient

[ukinau]

Description

Previously SSH NativeClient grasp error about session.Wait(), session.Run() and always return nil indicating succeeded even if command execution on the destination was failed.
But the error of session.Wait, Run is that definitely we need to feedback to user. so this PR changed the code to return error if session.Wait, Run failed in Shell func which is used by docker-machine ssh.

Related issue(s)

#4549

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "fix-#4549" git@github.com:ukinau/machine.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[shin-]

LGTM, thank you!