Handle API error(s) in Tomtom matrix. Fixes #6884

I previously tried to correct a circular dependency problem in
a057276129b1f62b811743db3b8f867a05241ed3, but that didn't fix it (it was
intermittent, and came back).

After some wrangling, I've now deduced with some confidence that the
problem comes from `RouteWayCostCalculator`, which used
`TomTom::Matrix::RemoteError`. From the way it looks, this seems to mess
up the Rails autoloader since `tom_tom.rb` will try to load the `Matrix`
class from the `TomTom.matrix` call above. Or something.

In an attempt to fix the circular dependency error for real this time,
move the error class to a completely separate module from `Matrix`, and
refer to this when we need to use the error class.

Refs #6884

If there's an API error, we shouldn't update the route's `costs` field.
Let's say we've already calculated some costs for a route A. We then edit
and re-save A, which triggers a recalculation of the costs. Now the
TomTom API responds with an error. We don't want to overwrite our
existing costs with an empty array because they could still be useful.
In this case, we should instead keep the existing costs we already had.

To achieve this, move the `RemoteError` rescue into
`RouteWayCostCalculator`, leaving the error unhandled in
`TomTom.matrix`.

Refs #6884

Calculate routes using the TomTom matrix API. Update the spec's HTTP
request stub accordingly (note that the stub is still fake).

Refs #6222

This new method will launch a worker that takes the route's `StopArea`s,
converts them to `WayCost`s, sends them to the TomTom API to calculate
distance and time costs, and saves those costs to the route's `costs`
field.

Refs #6095