Bogus "New object was not saved, no error provided" message

Once upon a time, all errors detected during migration were thrown as exceptions, so if after import we had no ID for the new object, we would generate this message as a fallback. Now, we can have errors saved without throwing an exception (notably on file migrations), and in those cases we get the "New object was not saved, no error provided" message when there really was an error provided. So, we should actually check the message table and see if anything is there...