Event handlers should be able to abort Plan execution on error

[vcschapp]

Currently httpx blindly continues the Plan execution after running the handler chain for a given event, e.g. BeforeAttempt.

This is naïve, because it assumes a handler can't run into an error itself, which is clearly wrong. A handler experiencing an error might theoretically want to/need to abort the Plan execution.

Handlers should be able to populate the value of Execution.Err and:

• If it is non-nil after a specific handler, the handler chain execution should possibly stop. [Validate if this makes sense.]
• If it is non-nil after a handler chain, the Plan execution should probably stop and the execution should fail.