Make errata more visible on the website

[mbollmann]

It's really hard to spot that a paper has an erratum on the website. For example, how long does it take you notice that 2021.emnlp-main.41 has one?

A simple improvement would be to turn the erratum link into a button, similar to what attachments have. Maybe we also want it to appear as a big button on the right-hand side.