--fail-on-error

Let's add this command line option. If it's provided, `pdd` should stop processing after the first mistake. If not, it should print the error and continue processing the next file. Thus, if this option is NOT provided, `pdd` should not fail no matter how many errors it meets.

Start at the pdd command-line entry point and review how existing options are parsed and how files are processed. Add --fail-on-error so the first mistake stops processing, while the default behavior reports errors and continues without failing; add or update tests for both modes.

Written by the indexing model from the issue text.