More informative log messages when parsing themes

[genetyx8]

When parsing themes, current messages aren't super helpful for debugging, e.g.
15:07:50] [Render thread/WARN] [minecraft/TextureManager]: Failed to load texture: saoui:textures/sao/guiedt.png
[15:07:17] [Worker-Main-12/INFO] [STDERR/]: [gnu.jel.debug.Debug:check:71]: Stack mismatch when compiling conditional
Both of these occur when loading a world with the hex theme.

It would be useful to systematically have information like the file name and line number for errors such as these (but really having them for every step of the theme loading, perhaps as a debug option would be great)

[Bluexin]

Many improvements have been added to the logging over the years, and only usage will help us know whether that's enough. The loading system has also been refined to isolate issues as best as we can and otherwise improve the theme dev experience.

I will keep this issue open as a reminder to check on the state of logging once the first new test versions go out.