fix error formatting bug

[icecream17]

As seen in https://github.com/metamath/set.mm/actions/runs/6345667560/job/17238038357?pr=3522 the "__" is interpreted as the markdown bold syntax, so this should be escaped

EDIT: This is kinda weird since there are no underscores at the end of the message...

[digama0]

o.O I had no idea there was markdownish formatting there. I assume it's something custom provided by the snippets library, I don't think it is actual markdown because markdown is HTML and that doesn't make too much sense in a CLI output.

[digama0]

Here's the formatting logic. Apparently it just handles __ specifically, as toggling emphasis. (Maybe we should use that sometimes.)

[icecream17]

Hmm, then I'm not sure this would work. This might just display as \_\_, at least that's what the code and rust-lang/annotate-snippets-rs#53 suggests

[digama0]

Does a zero-width space work?