Introducing "error page" if no edition exists (prevents failing)

[riedde]

Description, Context and related Issue

HTML page added for this case. It is analog to the page of multiple editions, but with a message that no edition exists. If no edition is found the controller will lead to this page. This prevents a total fail of the app and looks much better ;-)

Refs #335

How Has This Been Tested?

On local machine with docker and exist-db 6.2.0.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation Update

Checklist

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have performed a self-review of my code
• [x] I have read the CONTRIBUTING document.