Some updates in the CRAB2-to-CRAB3 translation code.

1) Use default name not only for CRAB3 config, but for CRAB2 config as well.
2) Remove 'serverUrl' and 'instance' parameters from the CRAB3 config.
3) Add translation of 'pyCfgParams' parameter.
4) If splitting parameter can not be deduced from CRAB2 config, don't set it to None, but to a string with a message saying that the splitting mode should be defined as either LumiBased, EventBased or FileBased.
5) Add translation of DBS shortcuts used in CRAB2 to the full DBS URL.
6) Fix typo: 'PrivateMC' --> 'privateMC'
7) Add function to retrieve the error message.