Refactor Observable to directly return value instead of Maybe

There were some cases where the implicit Maybe was problematic, because
it was impossible to guarantee a value. This also improved the
`runQuery` return type (returning only an `Either` or an `Either e Maybe`
is semantically easier to understand than the old `Maybe Either`.

This introduced a lot of necessary changes, but the result is a better
`runQuery` API: Every Query is now answered with either an exception
detailing why the query failed, or with a result.