Use system font on error pages

[AntonKhorev]

I opened #5131 and put a number of changes to error pages to see which of them are acceptable. The discussion there however has been about making more changes. I guess that means nobody objects to the ones I made and we can have them merged.

This PR changes the font of error pages to a default system ui font. Bootstrap does the same thing except also adds fallback fonts for older browsers that don't understand font-family: system-ui. We don't need a serif font for a text this short because we don't use serif fonts even for longer texts on the website.

Addresses this from #3532:

It also looks incredibly ugly, with loads of whitespace and the default browser font.

[tomhughes]

My PR hasn't been merged in two months so it must be good is an interesting take.

That said I'm not totally sure why I didn't merge it - possibly I was concerned about adding so much custom CSS code which we are generally trying to avoid.

Is there a reason not to use bootstrap here?

[AntonKhorev]

My PR hasn't been merged in two months so it must be good is an interesting take.

How else can I get changes from that PR to be discussed instead of being asked to make more changes like:

Is there a reason not to use bootstrap here?

Yes #5130 (comment)