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)