Updated findPath interface to return error structure instead of simple string.

Failed response now looks like:

{
    "correlationId": "03892717-684f-4ac2-b773-dcfb7192ba97",
    "status": "FAILED",
    "path": [],
    "m.findPathError": {
        "code": "00702",
        "label": "STP_RESOLUTION_ERROR",
        "description": "Could not resolve STP to a managing NSA
(urn:ogf:network:pionier.net.pl:2013:bi-ps-999?vlan=1780).",
        "resource": "dstStpId"
    }
}