#308 Allow for continuing step on error

[Terry O'Neill]

Current whenever there is an error during any step at best the step will need to be stopped and restarted. As some of the steps can take a very long time (e.g. the normalisation step) this can add significant time to the processing. In many cases it would be much more desirable to cater for retrying the current part of the step and continuing. At a minimum this should include steps that process over many files and allow for such errors as file not found errors.