Fix the DeleteAccount handler to return 501 HTTP status code because
it's what corresponds for a "Not Implemented" status.
Add a black box test for the DeleteAccount to ensure that always return
an error response because, at this time, we don't allow to delete
accounts through the API.
This test was not added to the corresponding commit
https://review.dev.storj.io/c/storj/storj/+/2712 due to the rush to
fix it.
Change-Id: Ibcf09e2ec52f182a8a580d606c457328d94c8b60
