Revision 37396

From Gerri:
- Fix a problem with the registration of missing files in the 'MissingFiles'
  list. The files that could not be open during processing were not properly
  registered in the list (only those found missing during validation or
  giving problmes durign reading were correctly added).
- Add method TProof::ShowMissingFiles() to facilitate the display of the
  list of missing files.
- Add method TProof::GetMissingFiles() to get a TFileCollection (dataset)
  with the missing files for further processing.