OK, partly answered one of my questions. From main Ekos screen I pressed "Logs" button. I configured verbose file logging for INDI and focuser drivers. I then pressed "Open Logs Directory" and found the following appear in a file after trying to reconnect the piface.

[2019-02-09T14:09:27.137 CST DEBG ][ org.kde.kstars.indi] - PiFace Focuser : "PiFace Focuser 1 device is not available. "
[2019-02-09T14:09:27.138 CST DEBG ][ org.kde.kstars.ekos] - PiFace Focuser is disconnected.
[2019-02-09T14:09:27.147 CST DEBG ][ org.kde.kstars.indi] - < PiFace Focuser 2 >: < DEBUG_LEVEL >
[2019-02-09T14:09:27.168 CST DEBG ][ org.kde.kstars.indi] - < PiFace Focuser 2 >: < LOGGING_LEVEL >
[2019-02-09T14:09:27.171 CST DEBG ][ org.kde.kstars.indi] - < PiFace Focuser 2 >: < LOG_OUTPUT >
[2019-02-09T14:09:27.186 CST DEBG ][ org.kde.kstars.indi] - PiFace Focuser 2 : "PiFace Focuser 2 device is not available. "
[2019-02-09T14:09:27.187 CST DEBG ][ org.kde.kstars.ekos] - PiFace Focuser 2 is disconnected.

So, here is the original error message I was asking about. However, non of the critical mcp23s17_open STDERR messages are going here.

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