How do I track error pages so they show up in my reports? | Google Analytics Help

Sometimes visitors will reach a 404 or “Not Found” error page from your site. This may happen when they type in a filename that they think is available on your site, or reach a deleted or non-existent file or directory from an obsolete link on one of your pages.

It’s valuable to see these requests so you can learn what visitors are looking for, and consider adding new content or fixing the broken link. Please follow the steps below to obtain reporting on missing or error pages [...]

Read the full article at: Google Analytics Help