Revisit the structure of the CONFIGURATION_ERROR

[mirceaulinic]

Introduced in #168, we might need to evaluate on the long run if the current structure really makes sense cross-platform:

"system": {
    "configuration": {
        "error": true,
        "message": "Interface must already be defined under [edit interfaces]",
        "path": "[edit vlans VLANTEST l3-interface]",
        "statement": "l3-interface vlan.666"
    }
}