GeezerVison

* Add Clinic.warn() and Clinic.fail() methods.
* Add filename parameter to BlockParser constructor.
* Add BlockParser.fail() method.
* Add DSLParser.fail() method.
* Pass lineno parameter to DSLParser.format_docstring(): more
  accurate error reporting.
* Convert a few DSLParser static methods to regular method to be able
  to call self.fail().
* Language.render() clinic parameter is now required: None is no
  longer accepted.
* Remove warn() function.
* Remove global variable 'clinic'.

fail() and warn() methods get filename and line number from the
instance on which the method is called, rather than relying on a
global variable 'clinic'.