SUP-16159 Improve WebSocket error handling

[yrucrem]

Abstract

Errors from Vert.x when creating an eventbus WebSocket where not handled which caused a NullPointerException during event creation. Typical errors like invalid event addresses are now handled, and multiple registrations of the same address no longer lead to errors.

Checklist

General

• Added abstract that describes the change
• Added changelog entry to /CHANGELOG.adoc
• Ensured that the change is covered by tests
• Ensured that the change is documented in the docs

On API Changes

• Checked if the changes are breaking or not
• Added GraphQL API if applicable
• Added Elasticsearch mapping if applicable