ssh: better error reporting on SshSession errors

[koreno]

We fail so frequently on various ssh configuration problems, and yet the error messages are mostly uninformative:

01:59:07 | No stderr result detected. Does the remote have Bash as the default shell?
...
01:59:07 |   */plumbum/machines/session.py:277 ................................ run >> return run_proc(self.popen(cmd), retcode)
01:59:07 |   */plumbum/commands/processes.py:292 ......................... run_proc >> stdout, stderr = proc.communicate()
01:59:07 |   */plumbum/machines/session.py:136 ........................ communicate >> msgerr) if name == "2" else SSHCommsError(msg)

It seems that the code was not ready for various other return codes

[coveralls]

Coverage decreased (-0.05%) to 29.093% when pulling b600191 on ssh-machine-errors into 99a9e6e on master.

[coveralls]

Coverage increased (+4.2%) to 33.374% when pulling b600191 on ssh-machine-errors into 99a9e6e on master.

[henryiii]

(retriggering the CI)

[henryiii]

This seems to break PyPy?

[henryiii]

I'm happy to try to move to GHA which will upgrade PyPy (this is 7.1.1-beta ?), then we can investigate. It might be that you can't do multiple inheritance on PyPy for the exceptions?

(But I'm currently moving, so might be a week or two)