PBCore Zipped nil:NilClass error still ingests asset

[ekemeyer]

Describe the bug
When using PBCore Zipped ingest, xml from GBH throws the error
undefined method `first' for nil:NilClass

This error happens when children are missing a mediaType value. This is not required for PBCore and so it should accept the xml.

UPDATE: Discussed with Rebecca, Peter, etc. and it was decided that mediaType should remain required (especially if we're moving ahead with not requiring instantiationPhysical (format), so we still need to deal with the secondary issue...

A secondary issue is that instead of ignoring the xml ingest entirely, the asset is ingested and the children are ignored (physical instantiations, digital instantiations).

To Reproduce
See batch 3914 summary. See any of the 101 assets that failed due to undefined method `first' for nil:NilClass error. They were all partially ingested into AMS2.

Expected behavior
Appropriate and clear error is thrown and nothing gets ingested.