Content Pack referencing deleted stream can be created

A Content Pack with entities that reference deleted streams can be created without any indication of problems.
The created content pack will fail to be installed every time, and the error presented to the user in both the UI and logs does not make it clear what the problem is.

## Expected Behavior

- You should not be able to create a content pack that will always fail to be installed, or without at least some feedback to the user
- Upon failed installation we should provide better error reporting

## Current Behavior

Users can create a content pack with entities that reference deleted streams. Installing this content pack will always fail and the error reporting is not very helpful:

## Possible Solution

Disallow creating a content pack that with references to non-existent streams.
Better error reporting.

## Steps to Reproduce (for bugs)

1. Create a dashboard with widget
2. Scope widget to a stream
3. Delete the stream
4. Export the dashboard in a content pack
5. Install the content pack

## Context

Customer experienced this issue and escalated to support.

## Your Environment

Specifically experienced exporting from Graylog 6.1 and installing on 6.3, however this is reproduceable fully on 6.3+,