SSHMachine Exceptions should be made visible at same level as associated class

[Michael Wilkinson]

I have code like:

from plumbum import SshMachine

try:
    remote_machine = SshMachine(...)
except SSHCommsError:
    ...

I would expect the Exception to be visible at the same level as the class it is associated with i.e.
from plumbum import SshMachine, SSHCommsError
Instead it is currently necessary to dive down:
from plumbum.machines.session import SSHCommsError

[Henry Schreiner]

I'd be okay with that.