The failure is a result of the presence of DTD's in the bundle (from the source Gateway), that don't have a public Identifier.
You can identify the offending DTD's by doing the following:
1. Login to Policy Manager as an administrator
2. Navigate to Tasks > Global Settings > Manage Global Resources
3. Sort by "Type" and look for any resources of type "DTD" that have a blank value under the "Details" column. These are the offending DTD's.
The current solutions are:
1. Add a public Identifier to the offending DTD's in the source Gateway via Policy Manager, then re-run a migrateOut
2. Remove the DTD's in the source Gateway via Policy Manager, then re-run a migrateOut.