Feature: exiftool error handling

[fxstein]

Need to implement an exiftool error handling strategy. While some errors like skipping existing files are ok in most cases, other errors are not. This applies to all calls of the exiftool but specifically where followup actions like --clean depend on the successful completion of a prior exiftool step.

This was raised by @Giga in #18. At the minimum we need to capture the exit status of every exiftool run and make it available as part of other logic for decision of whether to proceed in the goprox logic or not.