Handle missing data in objstorage

Currently, when a Content is missing from the objstorage (because it had been manually erased before or in a case of corruption), we should still be able to remove the associated Content object. Currently it crashes with ValueError(f"Unable to retrieve data for {swhid}").

1. This exception should better handled instead of displaying a stack trace.
2. A command-line flag should be able to relax this requirement and then create a Content object with no data in the recovery bundle (so it can be removed later).