Should ContentPackService#listAllEntityExcerpts fail on error with single Entity?

## Expected Behavior
When single entity, accessible by single entity facade, has problem (i.e. malformed entity was stored in Mongo DB), we should probably not fail completely the call to ContentPackService#listAllEntityExcerpts, but rather return limited list, with successfully created entities.
(may need discussion)

## Current Behavior
ContentPackService#listAllEntityExcerpts fails as a whole.
Functionality using this method fails as well (i.e. start page elements).

## Possible Solution
Not fail completely the call to ContentPackService#listAllEntityExcerpts, but rather return limited list, with successfully created entities.
Catch exceptions and react to them.

## Steps to Reproduce (for bugs)
One of the examples - HS-2290625939

## Context
HS-2290625939

## Your Environment
* Graylog Version: 5.2.3