/api/packages/versions/newUploadFinish logs warning if package is already uploaded

[sortie]

The /api/packages/versions/newUploadFinish API logs a warning if a package is already uploaded:

pubserver.shelf_pubserver: An error occured while finishing upload

Error:
    Exception: Version 0.6.0 of package exifdart already exists.

I don't believe this matters enough to us that we log a warning, the uploader gets the warning and can figure out the problem. Can we demote this case to an info? Of course, if any other unexpected upload error happens, we should log a error/warning.

[isoos]

I think the log event comes from package:pub_server, from this line. There seem to be legitimate cases where this event warrants a logger.warning.

However, if there were special exception types for errors that are supposed to be corrected by the user, the same code block could differentiate between the log levels.

Filed dart-archive/pub_server#17