Not to be obstinate, but I think I'd rather see a nice error message naming the file and the reason for the failure ("file is read-only"). The users of Grammatica are developers and as such they are less likely to do these things by mistake. For ordinary desktop users (on Windows), however, I'd also opt for some "intelligence" in the program.

Also, I suspect this might happen if you try to overwrite files on a shared volume where you only have read access. In that case, we can't really changed the file to read-write anyway...