Add custom error page for internal server errors

What does this MR do?

This error page is shown when we have an internal error. To prevent the stacktrace from being shown, I've added an error page.

I've also added a 404-specific page, for page-not-found errors, since the message then slightly differs.

Closes #75 (closed)

Screenshots

after

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.