improve warnings and errors when the dep is a checkout

[tsloughter]

There are at least two warnings/errors that are confusing messages when the reason for the message is a dep being moved to a checkout:

In rebar_prv_lock if a dep is moved to a checkout it prints the same message as it does if a dep is removed from the config:

%% TODO: don't output this message if the dep is now a checkout
rebar_utils:info_useless(OldLockNames, NewLockNames),

In rebar_prv_upgrade it prints an error if the user attempts to upgrade a dep that doesn't exis:

%% TODO: output a different error if the app is a checkout
?PRV_ERROR({unknown_dependency, Name});

If the dep is instead a checkout it should print an error saying that instead of that it doesn't exist.