[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.
Merge pull request #309 from sonatype/send-error-page-401-from-browser-request

[NEXUS-6189] take-3, override sendChallenge