Show any unexpected stderr output during __init__

[tkelman]

Should be somewhat less error-prone, to only hide exactly the expected warnings instead of every possible problem that might be going to stderr at load time.

[KristofferC]

I've had a lot of problems when redirecting the io, #40, #35, #14 so I'm a bit reluctant to touch that. Writing to a temp file seems to at least consistently work.

[tkelman]

This isn't touching it much, only enough to check what ends up there and show it if unexpected.

[KristofferC]

Oh sorry, I misunderstood the change.

[KristofferC]

Thanks 👍