Make IBackupTransport.finishBackup() also return an int code, since it too can

return TRANSPORT_NOT_INITIALIZED (in fact that's typically how it comes).

For consistency, make other IBackupTransport methods return int instead of
boolean, and handle accordingly.

Make initializeDevice() its own method instead of a flag on performBackup().
This will be needed when un-checking the settings box anyway, and is
conceptually unrelated to whatever happens to be the first post-initialization
backup we perform.  (Note that even if the init is sent separately from the
backup operation, the server will remember that an init has been done and
will *not* return NOT_INITIALIZED for the subsequent backup.)

Fix LocalTransport accordingly (trivial changes).

Handle failures more robustly in BackupManagerService -- most notably,
doQueuedBackups() was ignoring the result code of processOneBackup(), so
a NOT_INITIALIZED error would go past unseen (at least until the next
backup pass).  Keep track of error code returns more universally now.
(This includes finishBackup(), of course.)