errors/types.go: add a new UnprocessableEntityError
errors/types.go: add a new UnprocessableEntityError

To indicate that we have encountered an HTTP 422, let's introduce a new
sentinel type to do just that.

It will be instantiated and returned in a subsequent commit from
tq/basic_upload.go, and then caught down in tq/transfer_queue.go.

github/git-lfs,github/git-lfs,github/git-lfs