Raise exception if timeout occurs when starting the ssh client.

[danielroedl]

Issue

If the ssh client is not started within the timeout the start_client function exits without an error and without a client.
This leads later to a SSHException("No existing session") which does not allow any conclusions to be drawn about the cause.

Fix

On timeout raise a SSHException with a proper error message like it is already done in the open_channel function.