More precise error locations in src/parser/grammar.lisp

* src/parser/grammar.lisp (defrule/locations): inject
  `with-condition-translation' into :around options
* src/parser/package.lisp (package protocl.parser): added used package
  more-conditions
* protocl.asd (system protocl-parser): added system dependency on
  more-conditions
Decreased priority of comment in message-element in src/parser/grammar.lisp

* src/parser/grammar.lisp (defrule message-element): try comment last
  to allow injection of special syntax at the front
Disabled references to pb package in src/frontend/protocol.lisp

* src/frontend/protocol.lisp (load/text stream): disabled references
  to pb package
  (load/text sequence): likewise