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