WeBWorK: overwrite prior image files when unpacking an archive

[rbeezer]

Regenerating WeBWorK representations on Windows fails to update TikZ images, reported on pretext-support: each downloaded .tgz image archive extracts correctly, but the subsequent os.rename of image.tex/image.pdf/image.svg/image.png to the problem's image name raises FileExistsError on Windows whenever a prior run's file is present (POSIX replaces silently). The bare except then reports the failure as "did not contain a ... file" — for every problem and every format — and the archive is discarded, leaving stale images. os.replace() has the same signature and silently overwrites the destination on all platforms, so a Linux run is unchanged and a Windows run now overwrites prior generated images, matching Linux. In passing, the warning-message template moves adjacent to its single use.

Verified on Linux with the WeBWorK sample chapter against webwork-ptx.aimath.org: before/after runs produce identical image inventories, byte-identical representation files and .tex/.svg images, and pixel-identical .pdf/.png images (byte differences are per-request server rendering); regenerating into an already-populated images directory completes with no warnings.

Claude Fable 5, acting as a coding assistant for Rob Beezer

[rbeezer]

Looking for confirmation on a Windows machine....

[rbeezer]

For @sean-fitzpatrick

[sean-fitzpatrick]

I applied the change manually on my Windows computer, and got a clean run, with all the images where they're supposed to be. This seems to resolve the issue!