Revert "Merge pull request #311 from sonatype/401-error-page" due to build failures (cause: error page cannot be webresource, as it accepts only GETs).

This reverts commit 75f0a0e0b9db99953a4aa3ec3c03a6750fa24e21, reversing
changes made to 7cbf810e804fa9a88c84b324275f9487fbd2f10a.
[NEXUS-6189] take-3, override sendChallenge and if the request looks like its make from a browser, render an error page, otherwise perform default (send www-authenticate).  This makes the omission filter use-less so removing it.

Add error.html web-resource to handle generic servlet error-page muck
Add dispatcher REQUEST and ERROR to guiceFilter so that error page references can be resolved.

sendError to propagate 403 up to servlet container.
notes about why this is so fucking strange.