Add warning when watcher fails to send an update

[XAMPPRocky]

This is a simple fix to make debugging #682 easier, at least it should give us a better indication of where the failure is occurring.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 84ee31f2-dcd1-4447-93ad-5e6628731391

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.6.0-dev-9cb0d8b

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/684/head:pr_684 && git checkout pr_684
cargo build