Improve logging when restore fails to load Package Manager.

When the transport has no more packages to return, #nextRestorePackage()
returns RestoreDescription.NO_MORE_PACKAGES. Instead of logging that it had
been unable to get the package metadata, the code in BackupManagerService
was assuming that this was a valid package but one with an unexpected
package name, and hence logged a confusing error message.

Bug: 37695157
Change-Id: I75f2123d2bb062ad04003d7ef2a3a25d9661338c
Test: manual