this MOUNT entry is used to substitute a response message when an
error occurred.
It is applied If the response status code for the request shows an error (4xx
or 5xx), and the status code is in the listOfCodes if it is specified.
For example with MOUNT="/path/* file:/tmp/autherr.cgi onerror={401,407}",
the output from /tmp/autherr.cgi is sent when authentication error occurred
in access to URLs under "/path/".
