Comment 10 for bug 1538333

* Fixed bug #1538333 Assertion error in manifest.py: assert filecount == ...
  - Made sure to never pass .part files as true manifest files
  - Changed assert to log.Error to warn about truncated/corrupt filelist
  - Added unit test to make sure detection works
  - Note: while this condition is serious, it will not affect the basic backup and restore
    functions. Interactive options like --list-files-changed and --file-changed will not
    work correctly for this backup set, so it is advised to run a full backup as soon as
    possible after this error occurs.