_numpy_from_sample: Improve error reporting

Print a more useful message when wrong type passed to `_numpy_from_sample`.

I ran into this when mistakenly passing a region as the first argument to
`stbt.ocr`.
stbt._set_config: Don't fail if ~/.config/stbt/ doesn't exist

This happens if the user hasn't created `~/.config/stbt/` and hasn't
specified `$STBT_CONFIG_FILE`.