Remove custom error page

To be merged once main error page does not include error info.
Should we say something more along the lines of "Our system has notified us of this problem" otherwise we're going to get a load of user of emails saying "Page x doesn't work". Seems redundant?

Rails used to do this [by default](https://github.com/rails/rails/commit/1606b6c7e4af5ebc0bc705f3c4e4ea63b038ef14) – the older phrasing should work in our case?