Reject invalid coordinates with a helpful message

[zerebubuth]

A web mercator tile z/x/y can only exist if x or y are at least 0 and less than 2**z. We can use this information to quickly respond to requests outside of that range for either coordinate (i.e: without needing to check S3). We can also respond to requests where z is negative or larger than some configurable value.

We should still respond with a 404, but can include a helpful message so that clients get more information about why the tile isn't present.