I'm going to have to disagree with most of this.
Whilst I know exactly what it feels like to spend hours debugging only to realise that it's a stupid tiny little mistake that I made, the thing is, it can happen anywhere. For example, realising that an object is behaving strangely but turns out it was on an incorrect layer that had parallax scrolling. Once you have fixed the mistake though, you would probably remember in future to check the layering right away if another glitch were to occur.
I believe the concept I'm trying to get across links with the problem you have, Whiteclaws, with the function being typed incorrectly; since you spent a hellish long time trying to figure out the problem and eventually did, you will most likely look through and verify function names if you realise code is not working correctly from now on.
Anyway, in my humble opinion on the suggestion itself, I'd find a separate "Dropdown list" for functions personally not very useful, and having an error message occur when typing would cripple the Function tool. What if, like Ashley said, you wanted to call a function that had't just been evented yet- you'd get an error and that would be counterproductive. Unless the error message is just a little notification that pops up then fades away, then I can see a little bit of potential in that idea. One problem I once had was I accidently made a new function that already existed and had no idea of telling how until my brain decided to clock on and remind me lol. (That taught me to note down my functions on a commentbox that I have in my capx to prevent future issues). Not to mention how would the Function dialog work if a user typed in variable stuff like ["Load_" & Player.Something]- since there's no definitive "Function" right there unless the game is running, how would the interface know to show an error message or not?
I have no idea why I wrote so much about this but yeah!