ability to silence warnings when using multiple repositories

When using multiple repositories, the assumption is that the repositories will be iterated in order and a dependency will be resolved from the first that succeeds. Failing to find a dependency in any given repository is not usually of interest if it is found in at least one. When dependency resolution fails, however, it is convenient to have this information to recreate each successive resolution attempt. Silencing these warnings in the positive case, if possible, would be helpful, but perhaps the call doesn't need to be made at all: I see multiple urls for "mirror_urls" in my pin file, where, for any given dependency, I expect only one to work. If the dependency was resolved once from a given mirror_url, could that information be stored, and that url be tried first?

An example of the error message I am referring to:

`WARNING: Download from ... failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException GET returned 404 Not Found`

Start by reproducing dependency resolution with multiple repositories or mirror_urls and trace where failed download warnings are emitted. Done should mean expected failures from repositories tried before a successful one can be silenced, while failed resolution still retains enough information to diagnose each attempt; consider whether a previously successful mirror can be preferred.

Written by the indexing model from the issue text.