sendError to propagate 403 up to servlet container.
notes about why this is so fucking strange.
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.
Merge branch 'master' into 401-error-page

Conflicts:
	components/nexus-core/src/main/java/org/sonatype/nexus/security/filter/authc/NexusHttpAuthenticationFilter.java
Merge pull request #311 from sonatype/401-error-page

RFC: 403 error page