rrd_daemon: Log strerror(errno) when rrd_mkdir_p() fails

rrd_daemon: Log strerror(errno) when rrd_mkdir_p() fails

Log strerror(errno) when rrd_mkdir_p() fails in order to know what went wrong
from a client application perspective.

In this particular case, rrd_mkdir_p failed due to the lack of inodes.
Sending a message with 'No Space left on device' allowed the application
to limit the logs in the code in charge of handling the
RRDTOOL_FILESYSTEM_FULL error, rather than flooding the logs with all
the failed queries.