Output valid JSON when invalid coordinates are received

[Kristian Evers]

>curl http://127.0.0.1:5000/v1.0/trans/EPSG:4258/DK:S34S/12.0,56.0
{
    "v1": Infinity,
    "v2": Infinity,
    "v3": null,
    "v4": null
}

Infinity is not allowed in JSON. Should be replaced by an error string or possibly an HTTP error code.