Adding an empty manifest (say because a glob matched nothing) should at least be a warning

While working on making the Mercurial pyoxidizer.bzl more extensible, I had some files mysteriously not being included. After way too much debugging, I had problems in my glob() statements that were causing the imports to fail. Perhaps add_manifest(glob(...)) could warn or fail if you attempt to add an empty manifest?

Start with the pyoxidizer.bzl usage of add_manifest(glob(...)) described in the issue, then trace how an empty manifest is handled. Decide whether the requested behavior should warn or fail, and verify that files are not silently omitted when a glob matches nothing.

Written by the indexing model from the issue text.