Do::Halt - use Exception instead of StandardError

[gogiel]

Do::Halt is a StandardError. Because of that rescue block can unexpectedly catch it in a wrapped method. This problem has been reported at #72

If Do::Halt was a direct Exception derivative it would be nearly invisible to end-user.

Both ActiveRecord and Sequel catch Exception. While this indeed is a breaking change it shouldn't affect majority of users.

In Discourse discussion we agreed that this breaking change will be included in some future major release.

[nixme]

What about using throw/catch instead?

[flash-gordon]

@nixme using exceptions makes do notation transparent to transaction-like code (like one in AR and sequel), this is an important use case. We could have two versions of do notation with raise/rescue and throw/catch and let the user decide but that would be an overkill.