DJORNL parser: more `dry_run` options

[ialarmedalien]

• extend the dry_run option so that it can be called from the command line, e.g.

python -m importers.djornl.parser --dry_run

• alter the implementation of dry_run so that it prints dataset statistics and a list of errors, rather than dying with a list of errors without reporting stats.

• print out the stats in a more user-friendly manner