Checkmk Werk 13911: Improve RRD logging when path to DB is missing

ID: 13911 Title: Improve RRD logging when path to DB is missing Component: Livestatus Level: 1 Class: Bug fix Version: 2.2.0i1 Attempts to flush the DB when the path to the RRD database is missing would produce the rather unhelpful error "Error flushing RRD: Usage: rrdtool flushcached [--daemon|-d <addr>] <file> [<file> ...]" This fix removes the flushing attempt and silences the corresponding error in the log.