Description of problem:
Sequel of BZ#2096930 to improve the error message.
Non-admin user needs the `view_smart_proxies` permission to list/view smart proxies via API, but he is advised he needs `manage_capsule_content` instead.

Version-Release number of selected component (if applicable):
6.15.0

How reproducible:
always

Steps to Reproduce:
1. Try to list/show an external capsule via API for a user with no permissions.

Actual results:
$ curl -k -u unauth:unauth https://satellite.redhat.com/katello/api/capsules/2
{
"error":

{"message":"Access denied","details":"Missing one of the required permissions: manage_capsule_content","missing_permissions":["manage_capsule_content"]}

}

Expected results:
$ curl -k -u unauth:unauth https://satellite.redhat.com/katello/api/capsules/2
{
"error":

{"message":"Access denied","details":"Missing one of the required permissions: view_smart_proxies","missing_permissions":["view_smart_proxies"]}

}

QE Tracker for https://issues.redhat.com/browse/SAT-22358
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2257333