Prevent stat-uploading errors from polluting stdout

The error/warning messages that occur during stat-uploading are currently being printed to stdout, which breaks scripts that use pants output (e.g. "./pants goal list ::" chained with "./pants goal compile"). This RB is a fix for this issue.

Testing Done:
Local testing in a REPL

Reviewed at https://rbcommons.com/s/twitter/r/650/