avocado.test: Small error message fix (TestNotFoundError)

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins: Fix output plugins to include 'NOT_FOUND' status

We have introduced the new NOT_FOUND status, but haven't
updated the output plugins with it. Let's fix this bug.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #148 from lmr/not-found-fixes

Not found fixes