Add --quiet mode to CLI

It would be useful for various automation purposes if BuildStream could be silenced unless an error occurs. For normal CLI tools this could be achieved by just redirecting stdout to null and leaving stderr open, but BuildStream prints _everything_ to stderr. So, a --quiet flag that silences all warnings and informational output, and only prints errors, would be perfect!

Use-case: I have a script that I execute via `bst shell`. I want both stdout and stderr from the script, as well as errors from Buildstream, but I don't need BuildStream to print everything else (project warnings, pipeline status, etc).

Begin with the CLI path for `bst shell` and trace how BuildStream emits warnings, informational output, and errors; the issue names no source file or test. Done means a `--quiet` option suppresses BuildStream warnings and status output while retaining errors and both output streams from the script.

Written by the indexing model from the issue text.