In dokku/dokku#3602 a new error page for the 502 status code was
introduced.
However, this error page is not currently being used, and results in a
404 error being served instead, because nginx is trying to load a
non-existent file:
```
open() "/var/lib/dokku/data/nginx-vhosts/dokku-errors/502-error.html" failed (2: No such file or directory)
```
Fix this by copying the `502-error.html` file into the `dokku-errors`
directory upon installation.
05513ad75a