Vague error when no sensors found

[skalnik]

Recently a coworker loaded up AQI.WTF and got an error! Turns out they're in the Caribbean and there's no sensors near them within the bounding box we've setup with #85.

We should handle this state a bit better by either:

1. Expanding the bounding box
2. Showing a nicer error state where we say something like Hey, there's no sensors within Nkm of you!

Right now it just errors on this line: https://github.com/skalnik/aqi-wtf/blob/main/app.js#L116