Customising error messages is often overlooked, but it’s easy to tailor this experience for your website. See: https://salsadigital.com.au/somepage as an example.
In this exercise you’ll create a custom error message/page. The page could be a graphic, or a useful message.
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Create basic pages with an appropriate message for each situation (e.g. http://example.com/notfound and http://example.com/accessdenied\)
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Specify the error page 
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Go to Configuration → System → Basic site settings
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Type in the paths in the bottom section on Error pages
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Save configuration