Azure plugin: check permissions before deleting all blobs
Azure plugin: check permissions before deleting all blobs

Wrap the delete all in a permissions check for an extra layer of protection.
This may be an overabundance of caution, but it checks for permissions
AFTER it's already validated the nonce, mostly to add a notice.

mrgregwaugh/windows-azure-storage,mrgregwaugh/windows-azure-storage