putTile should not fail if initial GET times out

[rclark]

In a couple of high-write applications (e.g. https://github.com/mapox/mapbox-tile-copy), we're seeing a lot of failed putTile requests where the failure traces to 5s timeouts on the GET request that we make in order to compare buffers before doing an S3 PUT.

To me it seems that this is unnecessary, and that really any failure of the GET request within a putTile ought to be silently ignored. The buffer check is an optimization for minimizing PUT I/O and $$$ costs that shouldn't be allowed to cause 🔥.

cc @yhahn @GretaCB