Check for existing pids before ingesting

[bondjimbond]

Found in #122

If ingesting new objects AND assigning specific PIDs to them, if the PID already exists (whether in the batch queue or in reality), you get a "successful" batch created, but on ingest you get an error from Islandora Batch Ingest:

Unknown error: Failed to ingested [PID]

To avoid this error, IMI should check to make sure the PID being assigned doesn't already exist.