mruby-math: define and use `Math::DomainError`

Domain errors are detected as per ISO C99 and may differ from CRuby.


Should **mruby-math** follow CRuby behavior, even if it differs from C99?

One (I haven't checked all yet) example (where CRuby is wrong):
```c
Math.log(12, -1)  # should raise a domain error, but doesn't
```