Don't throw an exception from startNavigation

This is inside the `startNavigation` function. The current experience was for convenience and ease of a "clean" implementation but it can be confusing to users.

While most users won't ever hit it (it's hard to get a route without a location), throwing isn't great.

Possible solutions:
* Accept a third parameter: `UserLocation` of some sort
* Wait for an update from the location provider and capture that we are "pending" navigation