Should raise ParsingError instead of ArgumentError

[ksss]

I noticed that there are cases where obvious syntax errors are being reported as ArgumentError, so I have corrected this issue.

actual

$ bundle exec rbs parse --method-type -e '(foo` untyped) -> void'
bundler: failed to load command: rbs (/Users/ksss/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/bin/rbs)
/Users/ksss/src/github.com/ksss/rbs/lib/rbs/parser_aux.rb:12:in `_parse_method_type': negative string size (or size too big) (ArgumentError)

expect

$ bundle exec rbs parse --method-type -e '(foo` untyped) -> void'
-e:1:4...1:5: Syntax error: unexpected token for function parameter name, token=``` (tOPERATOR) (RBS::ParsingError)

  (foo` untyped) -> void
      ^

[soutaro]

👍