As described in ISO 15.2.30.

`FrozenError` is a subclass of `RuntimeError` which used to be
raised.  [Ruby2.5]

It's incompatible with CRuby, but it is required for mruby because
it cannot detect `break` outside of loops in the parsing time, but
in the code generation time where it cannot raise `SyntaxError`.

It overwrote the original definition in src/error.c, line 446.

Add `NameError#name`. Fix `NameError.new` and `NameError.initialize`. Enable 2nd argument for `NameError.new` in C API.