Error handling for `imagemagick` and `ffmpeg` should be improved

[agoose77]

Right now, we assume that any image conversion failure is due to missing imagemagick. But, not only are multiple converters supported (such as ffmpeg), these routines can also seemingly fail for other reasons: https://discord.com/channels/1083088970059096114/1126886232060870678/1349240214782218251

https://github.com/jupyter-book/mystmd/blob/03d72de9f7ef3465a7a305ccec06145a3dc1473f/packages/myst-cli/src/transforms/images.ts#L500