Description of problem:
When attempting to run `hammer content-export` on a Content View containing a Repository that is either unsynced, or that has it's Download Policy set to On-Demand, an unhelpful error message is returned:
undefined method `first' for nil:NilClass
This could cause confusion for customers, as it looks like a serious error, but it is expected.

Version-Release number of selected component (if applicable):
6.13

How reproducible:
100%

Steps to Reproduce:
1. Sync some repository like Satellite Client, and ensure that the download policy is set to On Demand
2. Create a Content View, attach the Repo, and publish a version
3. SSH into the Satellite, and run the command: `hammer content-export complete version --content-view="<CV Name>" --version=<CV Version> --organization="<Org Name"

Actual results:
Error: Remote artifacts cannot be exported.
undefined method `first' for nil:NilClass
Is returned as the error message.

Expected results:
Perhaps only the first line, and not the undefined method line; could also give some better explanation for the cause.