Fix error recovery & error reporting

Step 1: remove existing implementation

OK  * Put reason parser in its own file
OK  * Remove menhir error management:
OK    - remove "error" token 
OK    - remove error messages infrastructure
OK    Now message defaults to "Syntax error"
    * Fix reported error location (when recovering or not)

Step 2: reintroduce recovery
    * Preprocess grammar:
      - check exhaustivity of recovery
      - produce a mapping of automaton states to automaton-items suitable for
        recovery
    * Instrument parser:
      - first, always complete the AST and drop user input (":'(")
      - second, introduce an heuristic for recovering based on location

Step 3: reintroduce error messages
    * Ask the crowd: 
        What should messages look like? 
        Which situations are tricky or counter-intuitive?
    * Make a testsuite representative of common syntax errors 
    * ... Design an analysis sufficient to produce the messages automatically
          :P