possible idea (using FM 14 and its new GetDirectory script step)
Line 1 allows for creation of a new folder, because you get the "New Folder" button in the dialog.
After line 2 you can PatternCount $$MyPath to check if it contains the predefined Folder name, and if it's not, issue an error message and start over (enclose lines in a loop which is exited only if patterncount is positive and OK was clicked or if Cancel in the error dialog).
Is there problem if silently make the folder if not exist?
OP does not want to use plugins... otherwise a one-liner script using 2 functions from the free Base Elements plugin would do the trick.
Making folder can be done without plugins on Windows.
Send Evnet [ "cmd /c md " & Quote ( Replace ( Get(DesktopPath) ; 1 ; 1 ; "" ) & "foldername" ) ]
There may be similar for Applescript on Mac.
Being a Mac guy I can handle the Mac side with Applescript. They are a PC shop so I might do an if statement based on platform and use your code but once again, if that is done after the attempt to export the file you still get the error message. Then the question is will trying to create the folder before attempting the write and the folder already exists produce an error you can't suppress since it could be a system level error.
Gonna try. The easier way would be simply suppress the error and then use the new Get Directory command and let the user select a new folder if the default folder doesn't exist. But so far it appears there is no way to suppress the error.