renamed mrb_exc_new3 to mrb_exc_new_str

- more descriptive name (there's no mrb_exc_new2)
- no wrapper (macro/static inline function) for
backwards compatibility because nothing besides
mruby itself seems to be calling this function:
<http://mruby-code-search.ongaeshi.me/home?query=mrb_exc_new3>
Merge pull request #1584 from cremno/rename-mrb_exc_new3

renamed mrb_exc_new3 to mrb_exc_new_str