floss: Client checks if manager is valid

Add a check to make sure the manager is valid before using it in
btclient. This change doesn't exit right away if it's invalid and
instead allows the next command to fail (RegisterCallback) which gives
the reason for the failure.

Bug: 202023404
Tag: #floss
Test: Try running btclient with no btmanagerd running
Change-Id: If772cc729afc3fad3e0abb8aa2351157521d0f1d