Add .pre_handle method to EventNotificationHandler

[xavdid]

Why?

In final testing, users have expressed interest in being able to centrally run code before any other handler executes. It can be used for centralized logging, event deduplication, and more. I don't want to go full middleware, but this felt like a reasonable way to dip our toe into the waters.

Because users are responsible for deduplicating events, this pre-handle hook (if present) should return a bool, signifying whether handling should continue.

If the function returns true (or is missing entirely), the handler proceeds like normal (running at-most-one other callback). If it returns false, no further callback is run and the original .handle() call finishes cleanly.

What?

• add the .pre_handle method
• & associated tests
• update some error messages with correct verbs & consistency
• refactored some internal error handling
• updated example

See Also

• http://go/j/DEVSDK-3249

[jar-stripe]

Approved to unblock, but see the comment