Move chunker and LessError into separate files

This begins the process of cleaning up parser.js
[see #1902]
Pass env to chunker for use in error messages
Fix LessError being used before being defined
Add missing semicolon
Fix incorrect variable name for error
Merge pull request #2022 from ForbesLindesay/2_0_0-refactor-chunker-and-less-error

2.0.0 refactor chunker and less error