add option to resume incomplete exports

[ctgraham]

JIRA Ticket: n/a

What does this Pull Request do?

By default, the create-islandora-bag command will overwrite existing bags with the same name. An option --resume is available which will instead accept the existing bags as-is, and resume processing with the next incomplete bag.

What's new?

Drush option --resume examines existing bags, and if present emits a message before proceeding to the next bag.

How should this be tested?

• Cancel a collection export of multiple objects mid-way through.
• Re-run the command with the --resume option, and observe that completed bags are untouched, and missing bags are added.
• Re-run the command without the --resume option, and observe that all bags are re-created.

Additional Notes:

This does move a late set of the $serialized_bag_path and $compression_type to an early statement for the sake of checking for the existence of the serialized output.

Interested parties

Probably no one, just documenting our change.