Description of problem:
Incremental update should error out gracefully when invalid id is provided for errata/package/puppet module

Version-Release number of selected component (if applicable):
Sat 6.1.5

How reproducible:
Always

Steps to Reproduce:
Attempt to do an incremental update with invalid puppet module id or package id or errata id.  The below example uses puppet module id:

# curl -k -H "Content-Type: application/json" -X POST -d  '{"description":"Testing incremental update", "add_content":{"puppet_module_ids":["bad_id"]},"content_view_version_environments":[{"environment_ids":["1"],"content_view_version_id": 19}]}' -u admin:changeme https://$sat6host/katello/api/content_view_versions/incremental_update

{"displayMessage":"undefined method `repositories' for nil:NilClass","errors":["undefined method `repositories' for nil:NilClass"]}


Actual results:
{"displayMessage":"undefined method `repositories' for nil:NilClass","errors":["undefined method `repositories' for nil:NilClass"]}

Expected results:
Proper error message should be displayed saying that the id is invalid.

Additional info: