Validates that given tile coordinates are within valid bounds.

[danpat]

Addresses #183

[TheMarex]

👍 looks good to me.

[daniel-j-h]

Why do we only throw an error for the tile parameters here?

Every other service (and the tile function itself) simply asserts on isValid, e.g. see:

https://github.com/Project-OSRM/node-osrm/blob/fix/183/src/node_osrm.cpp#L938

Can we make the behavior consistent? That is removing the assertion (which currently will crash Node in debug builds or quietly work / crash libosrm in release builds --- neither is an option) and throwing Javascript errors instead?

[TheMarex]

I think this is fine as we usually don't use IsValid because it doesn't give us diagnostics of what made it invalid. In this case there is only one source of errors once you established its integers.