Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Due to the accepted "monad of no return" proposal, return becomes an
alias for pure. Return can be a pitfall for newcomers, so we decided to
use pure instead.

Both were lost while filtering the repository

This is a requirement for multi-hop network connections.

This fixes ghci.

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.