Allow service methods to omit return value

[dahlia]

Currently service methods can't have void/unit/null return type. Forcing every method to return something may sensible when we didn't have errors (#38), but no more now. In certain scenarios, no news is good news and we can raise an error when something goes wrong.

A syntax I would suggest is to simply omit method's return type:

service foo-service (
         bar (text value) throws bad-news,
    text baz (text value),
);

One more thing to consider is whether to force methods lacking return value to have error.