htaccess: suppress 'File does not exist' error log messages

htaccess: suppress 'File does not exist' error log messages

Various bots, spiders, crawlers, etc. often make requests for
files that do not exist.  Various web clients also make requests
for several "well-known" files that may or may not exist on
the server.

When any of the files are not found a 'File does not exist'
message is spewed into the error log file.

Since these messages are generally uninteresting and only serve
to make it more difficult to see any real errors in the log
(there are a LOT of not found errors), suppress them.

Since this is a .htaccess file, the suppression can easily be
disabled temporarily while diagnosing a problem that actually
requires these error messages.  Since the access log continues
to record the 404 result, this will normally not be necessary.