WIP: 403 error page

[restored from #311 - needs reworking due to test failures in https://bamboo.zion.sonatype.com/browse/NX-OSST-75]

Change authz handling to allow 403 handling to filter up to jetty so that it can use servlets error-page.

Add a /error.html error-page to split out terse details.

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF15