floss: Make inotify watchers more resilient

Sometimes during boot, /sys/class/bluetooth won't be available when
btmanagerd runs. In such cases, we should poll the directory rather than
panic right away.

Also refactor some logs to properly reflect why failures are occurring.

Bug: 225434492
Test: Reboot test on ChromeOS
Tag: #floss
Change-Id: Ie4d0ea161270b269a9dde0431767b4d75892c1ec