Improved migration script (for 71X)

This is a re-submission of #2725 that added into the BOOSTIO branch the 55f785e commit -- but this time for 71X (as should have been in the first place since we wanted to have it in both branches).

  * Creates different output files automatically per account/GT
    imported and in a different folder (timestamp).

  * Checks for the CalledProcessError exception.