31f65c0 backend: run_cmd() to dump stderr for code != 0 without check

Authored and Committed by praiskup a year ago
    backend: run_cmd() to dump stderr for code != 0 without check
    
    We experienced some disk-limit, causing the appstream-builder failures
    (see the PR#2358 for the discussion).  So we applied a work-around and
    ignored the exit code (run_cmd() check=False) for a while.  Then we
    realized the stderr output is actually never dumped to the logs so
    nobody can ever notice the error.
    
    This commit assures that the stderr output is either printed to log
    (logger is not None), or at least into the exception message (when
    check=True).  The third option - "logger=None and not check" - the
    run_cmd() caller needs to handle the returnvalue.stderr explicitly.
    
    Merges: #2365