rate error request reporting

[isiyu]

Currently tile requests are not returning 429 errors for ratelimits.
Pass this on from the requests so unity client can properly handle.

@david-rhodes mentioned you wanted this to be an exception that can be caught?

[wilhelmberg]

I think throwing an exception is kind of harsh for communicating information about a request (rate error, network error, ...).

My thinking was more like UnityWebRequest does it:

• HasError boolean property that can be easily checked if an error occured
• Error property that is an object: if we choose this to be a string, an exception or a custom object (maybe wrapping an (inner) exception that has occurred) is yet to be discussed

[david-rhodes]

@BergWerkGIS Agreed, and forgive me for misrepresenting what I actually intended--which is to encapsulate errors as objects. Ideally, all errors could be represented as objects, not just rate request errors.

In other words, I like your thinking!