[#803] Warn users before deleting an organization

Deleting an organization deletes all the public and private datasets
belonging to the organization. Add a warning to the delete confirmation
about this.
[#1010] Catch NotFound on new resource when package does not exist

Catch ckan.logic.NotFound exceptions raised when trying to create a new
resource for a package that doesn't exist. This turns a 500 Server Error
into a proper 404 Not Found. Fixes #1010.
Merge pull request #993 from okfn/803-warn-before-deleting-org

[#803] Warn users before deleting an organization