Allow the user to know when the creep can't complete the path.

[dyc3]

Currently, when the traveler is unable to find a complete path to the destination, it shows the path in red. I think it would be useful to be able to get this information via travelTo's return value, or a separate method that we could add to the creep's prototype.

[SystemParadox]

You already can. The returned path has incomplete: true if the path was incomplete.

[SystemParadox]

Oh, except you have to access it in a bit of a roundabout way, by passing an object as the returnData option that traveller will then update with extra information about the path:

let returnData = {};
creep.travelTo(..., { returnData });
if (returnData.pathfinderReturn.incomplete) {
    // ...
}

It would be nicer if the incomplete property was at least available as returnData.incomplete.