NEXUS-5817: index resource fetcher should obey proxymode

ResourceFetcher (MI specific stuff) is constructed in DefaultIndexerManager in way to circumvent ProxyRepository (by accessing it's RemoteStorage directly), and hence, it circumvents all the logic that _would_ handle the case of ProxyMode forbidding remote access.

Added manual handling of it, simply saying "404 Not Found" (as MI is not smarter to handle some special events, but the outcome is same).

Issue:
https://issues.sonatype.org/browse/NEXUS-5817

CI:
https://bamboo.zion.sonatype.com/browse/NX-OSSF31