Create generic 500 page

**Is your feature request related to a problem? Please describe.**
Sometimes errors can't be displayed properly on a specific page, we should have a generic error page that can accept a custom error message to display. For example, when middleware functions fail, we may want to send the user to an error page, instead of attempting to load the next page with a mangled request.

**Describe the solution you'd like**
Create a generic 500.html error page/template that accepts a message that simply gets displayed on the page.

**Describe alternatives you've considered**
None

**Additional context**
None

Start by locating the existing error-page templates and the middleware functions that handle failed requests. Add the requested generic 500.html template and connect it so a supplied custom message is displayed instead of continuing to the next page; verify the error path renders the message correctly.

Written by the indexing model from the issue text.