Floss: Refactor le-scan command handler

This refactors le-scan command handler to take advantage of early return
at error to reduce nesting.

Also piggibacking this CL are trivial warning fixups in
socket_manager.rs.

Bug: 254870159
Tag: #floss
Test: Build Floss on Linux

Change-Id: Id705bda6577bd16e757d34b3688d039e0c3adb6e