Improve failure modes, fix DXT1 lookup in transparency mode

[burnhamrobertp]

Fixes beyond-all-reason/maps-metadata#567

When processing the DXT1 lookup on each block, there is a bug in color ordering, causing them to be swapped. This caused the 2-bit pixel indices to map to wrong colors, resulting in visible artifacts. This PR also updated the RGB565 conversion to use simple bit shifting instead of bit replication, which produces smoother gradients matching the reference python implementation.

This PR only updates the affected code, and does not make any of the necessary changes to publish the updated version to npm. Typically this would involve:

Updating package.json and package-lock.json with the new version
npm run build
npm publsh (whoever is running this will need to be authenticated with npm and have publish rights for the original npm package)

[burnhamrobertp]

Example output (I can't remember the mipmapSize I used for this particular run, but it was very low, when artifacting aught to be at its worst; 8? Maybe 4?

[burnhamrobertp]

For full transparency, this PR does change more than what is strictly necessary, but much of what has been changed is in better keeping with typescript best practices, more robust error handling / fallback behavior, etc.

[p2004a]

(ignore ci, it need to be fixed on master)

[burnhamrobertp]

This PR shouldn't have been made off of master from my fork; it makes it hard to maintain synchronicity cleanly. I'll fix my local branches and push up this PR after I've rebased, using a dedicated branch