Suppress output on `--quiet`

Sergej Kostjucenko requested to merge cmichi-make-quiet-really-silent into master

Created by: cmichi

Closes #134 (closed).

Supersedes #133.

From the issue:

[…] investigate whether we should print progress updates to stderr as I believe cargo does (needs confirming).

I investigated and the cargo behavior is that progress output is suppressed entirely for --quiet, but errors are still printed to stderr. This is the behavior which I implemented in this PR.

Merge request reports