Can't close SMTP connection, when already disconnected.

[Sebastian Noack]

When the server has closed the SMTP connection (that happens rather quickly with most SMTP servers if you don't send any command for a few seconds), the exception SMTPServerDisconnected will be raised when trying to use the connection. That also happens when calling .quit(), like the SMTP backend does in its close() method.
So I have written a patch that catches SMTPServerDisconnected in addition to socket.sslerror, while closing the SMTP connection.

[Claude Paroz]

Thanks for the report. It would be nice to have a test for this. It's always a bit heary to simulate SMTP connections in the test suite, but I've been able to reproduce it with:

backend = smtp.EmailBackend()
backend.username = None # So as it does not try to login
backend.open()
SMTPBackendTests.server.stop()
backend.close()

[Hong Shuning]

I write the test case according to claudep's suggest.

[Claude Paroz <claude@…>]

In 4081042ef51c0e679c78f2d0d13514af3434ed13:

[1.5.x] Fixed #19134 -- Allowed closing smtp backend when the server is stopped

Thanks Sebastian Noack for the report and the initial patch.
Backport of 1b3f832ab7 from master.