Advise customers to run an fsck on their repos prior to migration

Summary

Customer in https://gitlab.zendesk.com/agent/tickets/143327 ran into an issue with large blobs causing an error in their migration. This brought about long delays and utilized many of our resources to debug.

We think that this would have been apparent early on if the customer ran a the integrity check rake task prior to attempting the migration.

Is it worth adding this to the pre-requisites of the migration? Something along the lines of:

It is good practice to verify the integrity of the repositories prior to migration. See the integrity check rake task.