Created attachment 929687 [details]
A traceback from /var/log/foreman/production.log.

Description of problem:
Issuing an HTTP GET request to /katello/api/v2/systems/:uuid returns an HTTP 400 response if the requested system does not exist. This is improper: an HTTP 400 response indicates that the request uses bad syntax. Fetching a non-existent system should return an HTTP 404 error, indicating that a non-existent resource was requested.

Version-Release number of selected component (if applicable):
This behaviour has been tested against qetello02.usersys.redhat.com, which is running Satellite 6.0.4. The following software is installed:

* apr-util-ldap-1.3.9-3.el6_0.1.x86_64
* candlepin-0.9.23-1.el6_5.noarch
* candlepin-common-1.0.1-1.el6_5.noarch
* candlepin-scl-1-5.el6_4.noarch
* candlepin-scl-quartz-2.1.5-5.el6_4.noarch
* candlepin-scl-rhino-1.7R3-1.el6_4.noarch
* candlepin-scl-runtime-1-5.el6_4.noarch
* candlepin-selinux-0.9.23-1.el6_5.noarch
* candlepin-tomcat6-0.9.23-1.el6_5.noarch
* elasticsearch-0.90.10-6.el6sat.noarch
* foreman-1.6.0.40-1.el6sat.noarch
* foreman-compute-1.6.0.40-1.el6sat.noarch
* foreman-gce-1.6.0.40-1.el6sat.noarch
* foreman-libvirt-1.6.0.40-1.el6sat.noarch
* foreman-ovirt-1.6.0.40-1.el6sat.noarch
* foreman-postgresql-1.6.0.40-1.el6sat.noarch
* foreman-proxy-1.6.0.28-1.el6sat.noarch
* foreman-selinux-1.6.0.9-1.el6sat.noarch
* foreman-vmware-1.6.0.40-1.el6sat.noarch
* katello-1.5.0-28.el6sat.noarch
* katello-ca-1.0-1.noarch
* katello-certs-tools-1.5.6-1.el6sat.noarch
* katello-installer-0.0.59-1.el6sat.noarch
* openldap-2.4.23-32.el6_4.1.x86_64
* pulp-katello-0.3-3.el6sat.noarch
* pulp-nodes-common-2.4.0-0.30.beta.el6sat.noarch
* pulp-nodes-parent-2.4.0-0.30.beta.el6sat.noarch
* pulp-puppet-plugins-2.4.0-0.30.beta.el6sat.noarch
* pulp-puppet-tools-2.4.0-0.30.beta.el6sat.noarch
* pulp-rpm-plugins-2.4.0-0.30.beta.el6sat.noarch
* pulp-selinux-2.4.0-0.30.beta.el6sat.noarch
* pulp-server-2.4.0-0.30.beta.el6sat.noarch
* python-ldap-2.3.10-1.el6.x86_64
* ruby193-rubygem-net-ldap-0.3.1-3.el6sat.noarch
* ruby193-rubygem-runcible-1.1.0-2.el6sat.noarch

How reproducible:
100%

Steps to Reproduce:
1. Issue an HTTP GET to /katello/api/v2/systems/:uuid, where the requested system does not exist.

Actual results:
An HTTP 400 response.

Expected results:
An HTTP 404 response.

Additional info:
This is an example of the message attached to one of the HTTP 400 error messages received:

    {
        u'displayMessage':
            u'Katello::Resources::Candlepin::Consumer: 404 Resource Not Found
            {"displayMessage":"Consumer with id abc123 could not be
            found.","requestUuid":"5d0f64df-e373-4273-b00e-6f1977bc3865"} (GET
            /candlepin/consumers/abc123)',
        u'errors': [
            u'Katello::Resources::Candlepin::Consumer: 404 Resource Not Found
            {"displayMessage":"Consumer with id abc123 could not be
            found.","requestUuid":"5d0f64df-e373-4273-b00e-6f1977bc3865"} (GET
            /candlepin/consumers/abc123)'
        ]
    }