Deletions during chunk movement can cause migrations to fail

If, while a chunk migration is in progress, objects inside of the chunk that's being moved are deleted on the donor shard, then the receiver shard can prematurely abort the migration, giving errors that look like:

Assertion: 16333:field names of bound { _id: ObjectId('5139739ec9b413112c01414b') } do not match those of keyPattern { a : 1.0 }

This does not compromise data, but prevents migrations from finishing, and impacts performance.