NEXUS-6863: Make regRoot fetch related failure propagate to the client

The change to make registry root fetch errors propagate to the client. They _usually_ stem from the fact that registry simply drops the connection on us before all the JSON arrives (currently 52MB, 99825 packages).

Originally, the MetadataStore was committing per-document to "salvage as much as possible", but this proved wrong, as partial data was presented to client. Now, the TX scope did change to commit "all or nothing". Memory/DB implications still remain to be seen. Also, the proxy MD service now logs and re-throws the exception, instead just logging it and continue. This results in HTTP 500 to appear on npm CLI side (good, what we want), but it seems npm CLI is stubborn enough to _retry_ the search. Still, there is console log indicating the problem.

Issue
https://issues.sonatype.org/browse/NEXUS-6863

CI
http://bamboo.s/browse/NX-NPMF15