Improve error 404/500 handling

The error 404/500 error handling solution provided by the archetype [0] is not very robust and seems difficult to achieve a "pretty" error page. Instead, a Apache ErrorDirective solution [1] gives the flexibility to use actual content pages and is quite easy to set up (once you know the syntax).

While this might not be a complete solution for a multilingual setup (the error code is using the `us-en` locale for all errors), it seems like a good start and the direction we can move towards.

[0] https://github.com/adobe/aem-project-archetype/tree/master/src/main/archetype/ui.apps/src/main/content/jcr_root/apps/sling/servlet/errorhandler
[1] https://github.com/adobe/aem-guides-wknd/pull/61

Start by reviewing the existing error handler under src/main/archetype/ui.apps/src/main/content/jcr_root/apps/sling/servlet/errorhandler and compare it with the Apache ErrorDirective approach in the linked WKND pull request. Determine how the generated archetype should provide pretty 404 and 500 pages, then verify the generated project handles those errors consistently; multilingual behavior is explicitly not complete.

Written by the indexing model from the issue text.