Prevent error level notifications from being sent when repositories return 404s

### Current Behavior

Notifications are received with level=error, scope=system and group=repository, when 404s are returned by repositories whilst fetching component metadata.

For rare/old components, it is common that they are not present in all repositories.

### Steps to Reproduce

1. Set a notification for level=error, scope=system and group=repository.
2. Find a package that is not present in all repositories
3. Wait (or trigger) a fetching of its metadata
4. Notice that a notification with level=error is received for all the configured repositories that do not have the given package

### Expected Behavior

These notifications should either not be sent at all, or be sent with level=info.

However, it would be interesting to send a notification with level=warning when the component is found in _none_ of the repositories

### Dependency-Track Version

4.7.0

### Dependency-Track Distribution

Container Image

### Database Server

N/A

### Database Server Version

_No response_

### Browser

N/A

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this defect was already reported