Construct 2 already logs the following message to the console if you call a function that doesn't exist:
"[Construct 2] Function object: called function 'blah', but no event was triggered. Is the function call spelt incorrectly or no longer used?"
If you're not already keeping an eye on the browser console while developing, then you should.
Dropdowns can make the workflow a little clunky sometimes; if you want to add a 'Call function' action then afterwards make the 'On function' event, if there's a dropdown which only shows existing functions, you can't add the 'Call function' action first.