Add Errr Logging filter

[macw]

Since upgrading to 2.15 I get hundreds of log file entries for

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Unable to verify referring URL. Please go back and try again.
every day. I've learned from this community that this error is new and most likely caused by bad bots.
And the only way to get rid of this error seems to be to fill htaccess with large swats of IP filters, which probably change next week anyway.

I hence suggest a filter feature for the error log that allows admins to exclude certain errors from being logged using a pattern or regexp.

The "Unable to verify referring URL. Please go back and try again." error is of no use or interest to me, but it pollutes the error logging, making it hard to find actual errors.