Surface image errors and warnings from HTML rendering in verifyproblem

[gkreitz]

Starts threading in Diagnostics into PlasTeX rendering to be able to better flag errors and warnings during tex -> html rendering. Also adds a check when converting pdf images to png, catching gs exiting with 0 without producing any png file.

After this PR, if rendering tex->html causes warnings/errors, those will be spotted by verifyproblem (and not just the generic failure message telling you to run problem2html to diagnose, even if that can still happen). This means that having large images is now a warning directly in verifyproblem, and failing to convert images is an error. I think this is a clear step up, but happy to hear feedback on if this ends up too spammy for some problems.

As a bonus, errors/warnings we ourselves emit in problem2html for tex->html are now colorful, like in verifyproblem.

This PR also deprecates --quiet in problem2html. That switch now only controls 2 print statements (use --log-level instead to control how much output you want). For obvious reasons, --quiet will not print a deprecation warning. :)

Fixes #470

[Matistjati]

I tested it on ~10 problems from PO where we have illustrations. No auto-generated illustrations triggered the warning, but real photos frequently triggered the uppercase warning. I think this is fine, since I have yet to see a problem where all that resolution is actually used. Perhaps this could've saved us from the 20k x 20k picture here 🙂 https://open.kattis.com/problems/investigatingfrogbehaviouronlilypadpatterns

[gkreitz]

Great, thanks for testing!

The uppercase warning has in some sense always been there (if one manually ran problem2html). I take it from your comment, that it's probably the right call to surface it in verifyproblem (I'm 99% sure the reason we hid it had more to do with earlier implementation limitations, rather than a choice)