Don't let bmgr leave a restore session hanging on error

Specifically, don't wait for the RestoreObserver to be informed that the restore
has completed unless performRestore() ran.  We were winding up in a case where
bmgr was hanging forever waiting on a nonexistent restore process instead of
calling endRestoreSession().

Also improve the documentation, explicitly calling out the need to call
endRestoreSession() even if previous operations on the session were
unsuccessful.

services/java/com/android/server/BackupManagerService.java

@@ -1725,6 +1725,8 @@ class BackupManagerService extends IBackupManager.Stub {
                     return 0;
                 }
             }
+
+            Log.w(TAG, "Restore token " + Long.toHexString(token) + " not found");
             return -1;
         }