Fixes #5613 - Fix unhandled sys errors BZ1084722

In hammer-cli-katello's system subcommand, when system-id given is
invalid, only a Bad Request message is presented to the user. This is
because the error messages being returned when a system is not found are
not being rendered using app/lib/katello/api/v2/error_handling.rb.

* Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1084722