I closed #28658 in favor of this.  For that issue, the exception was a UnicodeDecodeError instead of a configparser.Error.  Both should be caught.

Ingrid, sorry I never got back to this.  A few months ago, Serhiy wrote a warning function for the config(Handler) function that was included in a larger patch for the module.  I wrote a test_config that test both the warning function and the other new code.  This was for 3.6 only.

I am going to patch the lowest level IDLE function, IDLEConfigParser.Load, which currently calls ConfigParser.read, and catch exceptions and call the warning function right there.  For 2.7 and 3.5, I will make a minimal change, and test by hand (by editing errors into one of my user config files).  For 3.6/7, I will try replacing .read (which reads a list of files) with .read_file (which reads one file).