[cpackget] Improve robustness for handing index.pidx file

[ReinhardKeil]

The Problem To Be Solved

The index.pidx file is the central file for pack management. I recommend the following improvements:

• Make error message a bit more descriptive when index.pidx file is missing. Currently cpackget update-index --sparse reports:

  cpackget update-index --sparse
  I: Updating public index
  E: file not found
  

  Improved message suggestion:

  E: index.pdix file not found; use cpackget init command to retrieve it
  

• Update index.pidx even when the local time stamp is newer. This may need a bit more work, as it effectivly requires a checksum or equivalent in the index.pidx file.