MassTransit with RabbitMQ: When is a message moved to the error queue

I am using RabbitMQ version 3.0.2 & I see close to 1000 message in Error queue. I want to know

1. At what point messages are moved to the error queues?
2. Is there a way to know why a certain message is being moved to an error queue?
3. Is there any way to move message from error queue to normal queue?

Thank you