Update negative tests for test storage extension

The previous negative tests ware focused on the Rego behind the policy
logic. The improvement of errors in them can be solved by improving the
Rego logic. The updated tests are checking the Go code error handling
which is the priority.

Further Rego policy could be created and improve even better the error
information.

Signed-off-by: Georgi Michev <georgi.michev@vereign.com>