Better errors when determining detection method

[sellout]

Rather than just saying that we couldn’t determine the detection method, this indicates why and makes some suggestions. Probably even longer errors with concrete steps to remedy each case would be helpful.

This includes an alternative approach to the first bullet point in #61 – changing where we suggest that auto-dark-allow-osascript should be enabled (although this does not change the error that suggests it in the wrong place).

All of the logic here should be identical, it just splits it into multiple conditionals to avoid duplication and identify different failure modes.