Cleanup notASequence exception handling

A few changes:
* simpler, better-factored code
* fix function name to agree with error message
(maybe this should be "not an array or sequence"?)
* index < 0 is not used (0 = value, > 0 = index),
  and is safe anyway (ok to convert negative numbers to string)
* fuller list of ordinal exceptions:
(up to 19 should be plenty; we have up to 10 arguments in real IDLs,
CanvasRenderingContext2D.idl::drawImage
so also including 12 gives us plenty of room)
http://en.wikipedia.org/wiki/English_numerals#Ordinal_numbers

Followup to:
IDL compiler: Array and sequence types for methods
https://codereview.chromium.org/50623003/

Review URL: https://codereview.chromium.org/52063002

git-svn-id: svn://svn.chromium.org/blink/trunk@160912 bbb929c8-8fbe-4397-9dbb-9b2b20218538