IDL: Use user-friendly type names for error messages

This is a follow-up CL of https://codereview.chromium.org/690923004/.

It would be great to show "(A or B)" rather than "AOrB".

BUG=240176

Review URL: https://codereview.chromium.org/722523003

git-svn-id: svn://svn.chromium.org/blink/trunk@185195 bbb929c8-8fbe-4397-9dbb-9b2b20218538