Runtime: gracefully handle weird watcher issue

[begelundmuller]

Ran into an error where the watcher stopped because Rel received an empty target path:

{"level":"error","ts":1709713441.4184802,"msg":"Stopped watching for file changes","instance_id":"default","error":"watch failed: Rel: can't make  relative to /Users/benjamin/Documents/foo"}

Can't think of anything else that could have caused it than fsnotify emitting an empty event.