Adopt typed throws

[stefanceriu]

In SE-413 the concept of typed throws has been introduced to Swift.

We have designed Element X around Results particularly to be able to specify the exact type of a returning error but with typed throws in place we now migrate to using this language feature.

One complication here is the mocks we generate through sourcery. They recently added support for typed throws as well in krzysztofzablocki/Sourcery#1401 but our templates are heavily modified to bring thread safeness to the mocks. They were crashing on parallel testing and we fixed that by introducing dispatch queues on accessing properties.

Ideally, with swift concurrency now being more established, we would take the upstream templates (and typed throws support) and reimplement thread safety on top using the new mechanisms, perhaps even contribute it back to the community.