BZ#985218: Display more user-friendly error message when erasing non-exiisting cartridge

Before:

```
[root@xxx ~]# oo-admin-cartridge -a erase -n zend -v 5.7 -c 0.0.1
/opt/rh/ruby193/root/usr/share/gems/gems/openshift-origin-node-1.13.2/lib/openshift-origin-node/model/cartridge_repository.rb:199:in `erase': key not found: (zend, 5.7, 0.0.1) (KeyError)
	from /usr/sbin/oo-admin-cartridge:53:in `apply'
	from /usr/sbin/oo-admin-cartridge:137:in `<main>'
```

After this patch:

```
[root@xxx~]# oo-admin-cartridge -a erase -n zend -v 5.7 -c 0.0.1
requested cartridge does not exists (key not found: (zend, 5.7, 0.0.1))

```