Make full report work for draft v4 dependencies

I should "backport" these changes to the draft v3 keyword, too; the wording is
much nicer.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
DisallowKeywordValidator: set the schema path for subreports

It _is_ necessary so that fatal errors are reported at the correct schema level.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
NotKeywordValidator: set the path on subschema validation

Same as disallows: we want fatal errors to be reported at the correct path.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
AbstractFullReportTest: add a fatal-level keyword to test for "negatives"

We want to test whether fatal errors are correctly reported with keywords such
as "disallow" and "not".

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Test fatal error reporting with "disallow"

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>