Improve 500 error page

Created by: fonfon

Until #276 (closed) is implemented (or also additionally), we could make the 500 error page more helpful.

There could be an additional page that shows the last X lines of /var/log/plinth/status.log. The error page could show a link to it (Error-pages themselves should be as simple as possible, and make it extremely unlikely that another error occurs on them).

Users reporting an issue can directly include the information from there. This should reduce the issues with missing information (like #448 (closed)) where we have to ask users to provide the information from the log file manually.

Are there any objections to showing the last bit of this log-file in Plinth?