Move bindings/scripts/IDLAttributes.txt => bindings/IDLExtendedAttributes.txt

IDLAttributes.txt is not a script (it's a configuration file), so it doesn't
belong in scripts. Also renamed to IDL*Extended*Attributes.txt, as we're
using standard terms from spec.
Some fixes to message as well to make it more legible + cut-and-pasteable.

Review URL: https://codereview.chromium.org/70233004

git-svn-id: svn://svn.chromium.org/blink/trunk@161886 bbb929c8-8fbe-4397-9dbb-9b2b20218538