This change serves to declutter webhook-errors.log, which is
filled with too many UnexpectedWebhookEventType exceptions.
Keeping UnexpectedWebhookEventType in zerver/lib/webhooks/common.py
led to a cyclic import when we tried to import the exception in
zerver/decorators.py, so this commit also moves this exception to
another appropriate module. Note that our webhooks still import
this exception via zerver/lib/webhooks/common.py.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.