Improvements in duplicated content pack error message

## Description
Improve error notification when uploading a duplicated content pack by:

- Making error message clearer
Currently the error message states that it may not be valid JSON, and later adds some more details where one can see that the ID is already used. Separating the cases where the upload is not a valid JSON document and other errors would already help. Resolving the name attached to that ID would be even better.

- Remove escaped HTML tags from error message
In #4726 we enabled by default HTML escape on toastr to fix some XSS issues we had. Until we switch to another library to render user notifications, we should avoid using HTML in the notifications.


## Your Environment
* Graylog Version: Graylog 3.0.0-beta.4-SNAPSHOT (87afab1)