Silent failure / hang when `modem_path.exists()` is `false`

[paulyoung]

As discovered in https://github.com/pd-rs/crankstart/issues/41#issuecomment-1763542159.

This happens in at least 2 cases that I'm aware of:

1. PLAYDATE_SERIAL_DEVICE isn't set and the hard-coded value is used.
2. The device is locked.

[boozook]

In case of (2) device is locked, device is not connected so we can do nothing.