Provide `onRoutesRequestFailure` reason information

@LukasPaczos is already working on this.

Since Jul 5, 2021.

Currently, we're reporting back a `Throwable` with a tweaked message as a result of https://github.com/mapbox/mapbox-navigation-android/blob/306f2919494f7731a0b3515a9d3d9cae5bd0ad07/libnavigation-core/src/main/java/com/mapbox/navigation/core/MapboxNavigation.kt#L386-L389 https://github.com/mapbox/mapbox-navigation-android/blob/306f2919494f7731a0b3515a9d3d9cae5bd0ad07/libnavigation-core/src/main/java/com/mapbox/navigation/core/directions/session/RoutesRequestCallback.kt#L34

Clients can parse the string `Throwable.message` or print the whole thing but there isn't an easy way to tell the user the reason e.g. if it was offline or service error:

```
if ( /* is offline error */ )
// display prompt telling user to check network connection
else
// display user message apologizing for service being down
```

This can't be easily implemented with given information from the `Throwable`.

This ticket tracks the feature request of adding some kind of `Error` object including the `Reason` so that developers can use this extra information to act accordingly to their use cases (e.g. to determine what messaging to display).

`Reason`s may include offline vs service error, authentication failure, bad request, etc.

cc @mapbox/navigation-android