Error logging

[davetayls]

Currently you get an error like:

Fatal error: rework-vars: variable "--varname" is undefined
Error: rework-vars: variable "--varname" is undefined

When a variable is undefined, it would be nice if it gave a file and a line number a bit like you get on the error that gets thrown by the css parser itself.

{ [Error: /Users/[...]/index.css:19:14: property missing ':']
  reason: 'property missing \':\'',
  filename: '/Users/[...]/index.css',
  line: 19,
  column: 14,
  source: '...' }