Account.BulkDelete does not reauthenticate when running into 401 error

[majewsky]

Gopherschwift usually calls Reauthenticate() when the token expires, by recognizing 401 responses. However, when a bulk-delete fails because of expired token, 400 is returned and the 401 errors can only be found in the BulkError. Example log output from swift-http-import (which knows how to unpack the BulkError, every line after the first is one of the constituent BulkObjectError instances):

2018/06/07 13:03:26 ERROR: cleanup of 131207 objects on target side failed: 400 Bad Request: Max delete failures exceeded (+1001 object errors)
2018/06/07 13:03:26 ERROR: DELETE /repo/ubuntu/dists/xenial-proposed/main/installer-i386/20101020ubuntu451.15/images/hwe-cdrom/vmlinuz: 401 Unauthorized
2018/06/07 13:03:26 ERROR: DELETE /repo/ubuntu/dists/xenial-proposed/main/installer-i386/20101020ubuntu451.15/images/hwe-hd-media/boot.img.gz: 401 Unauthorized
2018/06/07 13:03:26 ERROR: DELETE /repo/ubuntu/dists/xenial-proposed/main/installer-i386/20101020ubuntu451.15/images/hwe-hd-media/initrd.gz: 401 Unauthorized
2018/06/07 13:03:26 ERROR: DELETE /repo/ubuntu/dists/xenial-proposed/main/installer-i386/20101020ubuntu451.15/images/hwe-hd-media/vmlinuz: 401 Unauthorized
2018/06/07 13:03:26 ERROR: DELETE /repo/ubuntu/dists/xenial-proposed/main/installer-i386/20101020ubuntu451.15/images/hwe-netboot/boot.img.gz: 401 Unauthorized
...