classify-sklearn orientation detector will raise confidence error

[nbokulich]

Bug Description
If working with reference databases with uneven taxonomic levels, confidence=disable will still raise an error with read_direction=auto, because the orientation detector must use confidence (to determine which direction is more confidently classified).

Steps to reproduce the behavior
See forum topic

Expected behavior
Should amend the error message that is raised to recommend workarounds.

Right now the error message is this:
ValueError: this classifier does not support confidence values

Should add a second sentence suggesting to use confidence=disable and set read_direction to bypass this error message.

References
forum topic