Add error control

[lwcorp]

There seems to be no error control, so if there's a failure it just returns an unknown reason:

In my case, the unknown reason was solved by:

1. Switch $bounce_mailbox_port to imap instead of pop3 and likewise use an IMAP port number rather than a POP3 one.
2. Define an actual manual hostname in $bounce_mailbox_host instead of just localhost.

But I assume other errors might happen as well, so it might be best to try to determine the error, at least in a general way, especially as this plugin might come as default.

[bramley]

"Unknown reason" wasn't to do with the mailbox port but the use of localhost. I have now included that in the plugin documentation.

But "Unknown reason" was exactly the case. The php function fsockopen() had not returned an error message

https://github.com/bramley/phplist-plugin-imap2/blob/master/plugins/Imap2/vendor/javanile/php-imap2/src/Roundcube/ImapClient.php#L1066

        $this->setError(self::ERROR_BAD, sprintf("Could not connect to %s:%d: %s",
            $host, $this->prefs['port'], $errstr ?: "Unknown reason"));

[lwcorp]

What a shame fsockopen returns no specific error. Don't you want to update the README here as well?

How about doing a simple #3?

[bramley]

I don't want to duplicate documentation in that way.

php-imap2 is an external package so I also don't want to change that just for this specific issue.

[lwcorp]

What if you detect localhost even before calling the plugin, for example in .php-cs-fixer.php?

[bramley]

I have added an explanation to the plugin documentation which I think is sufficient for the time being.

If this plugin became the basis of a long term solution for phplist to the issue of the removal of the IMAP extension then there could be some extra validation, such as the host name, protocol and port. I want to wait to see what that long term solution is going to be.