Backport 502 improvements to Community

[mtlynch]

All the guidance for HTTP 502 errors that we've added in https://github.com/tiny-pilot/tinypilot-pro/pull/967 apply to Community as well, so we should backport the changes to this repo.

Implementation notes

The easy way

The simplest way to backport these changes is just to copy 502.html from Pro, paste it on top of 502.html in this repo, and then commit the changes.

The fancier git way

Another way we could do it is the more git-native way:

1. Add TinyPilot Pro as a git remote to your copy of the Community repo
2. Use git cherry-pick to grab the specific commit associated with the 502 error message changes from Pro in a new Community branch
3. Create a PR for the new branch

That doesn't really win us anything because we're squashing the commit in the Community PR. If we didn't do squashed PRs, we could go this route to make things a little tidier in the commit history, but it's not a big deal.