test-ssh: stablize for platform-specific shell quoting Windows and OpenVMS use double quotes instead of single.

push: catch and process PushkeyFailed error We add a way to register "pushkey failure callback" that will be used if the push is aborted by a pushkey failure. A part generator adding mandatory pushkey parts should register a failure callback for all of them. The callback will be in charge of generating a meaningful abort if this part fails. If no callback is registered, the error is propagated. Catch PushkeyFailed error in exchange.