BagIt "download all" fails when a package contains missing data objects

[robyngit]

When a resource map includes data objects that are missing in object storage, BagIt (/mn/v2/packages/application/bagit-1.0/<resourcemap>) cannot create a zip file of the package. Instead it returns an xml response with error information:

<error detailCode="1420" errorCode="404" name="NotFound">
  <description>
    No system metadata could be found for given PID: foo.5062.1
  </description>
</error>

On the browser, the file is downloaded as a zip anyway, and can't be open ("unsupported format" on MacOS). This occurs for packages where placeholder Solr documents exist for missing objects, but the underlying content is absent.

Example affected package:
https://dev.nceas.ucsb.edu/view/urn:uuid:89e6238d-8335-484c-9be2-a6dfac22e6c0
BagIt endpoint:
https://dev.nceas.ucsb.edu/knb/d1/mn/v2/packages/application%2Fbagit-1.0/resourceMap_f6751583-3712-4a65-8b6b-ce86c172fab6

Expected behaviour

BagIt should generate a zipped package successfully even when some package members cannot be retrieved. Missing data files could be excluded or they could be included as empty placeholder files.