[swarming] Reduce error level for bad new task request
[swarming] Reduce error level for bad new task request

This is a user error which is correctly returned as 400, so there's no
need to trigger a stack driver report on this.

Change-Id: If4eafba5e0a3e1bfc32518ce56b1544a77bf8c70
Reviewed-on: https://chromium-review.googlesource.com/c/1351093
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py