Fix transaction extensions to handle failures correctly

[waiting-for-dev]

Overview

The intercepting_failure method was not handling the block's output correctly when it was a failure. It wasn't throwing :halt with the failure as the given handlers were raising and therefore the throwing part was never reached.

We modify the method to pass the failure to the handler and skip the throwing part, while extensions need to work in two phases:

1 - From within the transaction block, they intercept a failure a first time and assign it to a result variable available from the outer scope. After that, they raise to rollback the transaction but return the result variable after that.

2 - From outside the transaction block, they throw again the failure when the result is actually a failure.

We also make public the throw_failure method so that other extensions can create similar behavior.

We also add a warning in the ActiveRecord extension to make it clear that the :requires_new option is not yet supported for this approach. We should find a way to make it work in the future.

Closes #23

Screenshots/Screencasts

Details

[waiting-for-dev]

It seems like the job of writing an extension has become a bit more complicated with this change, but if that's what's necessary to provide correct behaviour, then that's fine :)

Yeah, unfortunately now everything becomes more cumbersome, but unfortunately I can't see any other way to workaround the two mechanisms breaking the control flow in sequence :/