Here you'll find how to create a Custom page not found message in blogger or blogspot.
The method for such preference editing is very simple,
Go to Settings in your blogger dashboard
Go to Search Preferences
See the Errors and Redirections part of the page.
choose 'Custom Page Not Found', clicking on the question mark will display the following message,
'Enter an HTML message that will be displayed on the Page Not Found page instead of the generic message.' which speaks for itself and there is no need to elaborate.
Finally Enter the text to be displayed, eg "Sorry, The page you're looking for is not Available." etc.
Here is a video depicting the complete steps about how to create a 'Custom page not found' message in blogger
Now you know how to create a