Fix inconsistency of message field in exception modules

[kentaro]

Neotomex.Grammar.ParseError and Neotomex.Grammar.ValidationError cause warnings as below.

iex(1)> try do
...(1)> raise Neotomex.Grammar.ParseError, message: "parse error"
...(1)> rescue
...(1)> e in Neotomex.Grammar.ParseError -> e.description
...(1)> end
warning: the following fields are unknown when raising Neotomex.Grammar.ParseError: [message: "parse error"]. Please make sure to only give known fields when raising or redefine Neotomex.Grammar.ParseError.exception/1 to discard unknown fields. Future Elixir versions will raise on unknown fields given to raise/2
  (neotomex 0.1.7) lib/neotomex/grammar.ex:106: Neotomex.Grammar.ParseError.exception/1
  (stdlib 3.16) erl_eval.erl:685: :erl_eval.do_apply/6
  (stdlib 3.16) erl_eval.erl:893: :erl_eval.expr_list/6
  (stdlib 3.16) erl_eval.erl:408: :erl_eval.expr/5
  (stdlib 3.16) erl_eval.erl:919: :erl_eval.try_clauses/8
  (elixir 1.12.3) src/elixir.erl:280: :elixir.recur_eval/3

"parse error"

It is because the field names in these exceptions are inconsistent. The field name description in defexception should be message instead, I guess.

This fix doesn't break any tests.