[parsing] Better error logging from package_downloader

[jwnimmer-tri]

In #18955, we have package_map.AddRemote(....) now, so calling package_map.GetPath("...") can sometimes kick off a download.

Because of the way it's implemented, if the downloader fails then the details of the error (e.g., "dns is broken") go to stderr -- our spdlog only gets a generic "download failed" message. See review thread for extra details.

We should be able to change the downloader to write its error message to a json file, and then have package_map.cc copy that back into spdlog.