error.c: better names for rb_exc_new family

* error.c (rb_exc_new_cstr): rename from rb_exc_new2.
* error.c (rb_exc_new_str): rename from rb_exc_new3.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41017 b2dd03c8-39d4-4d8f-98ff-823fe69b080e