Issue Summary:
When following the Google documentation [1] but modifying the code to include a notification ID on the notification object before creating it you get an error stating that the notification ID exists. However when running the gcloud command to describe a notification with that same ID it actually does not exist.

When looking at the source for this method we can see this error message is returned anytime the notification ID on the notification object is set to anything other than None [2].

The reference documentation [3] also states that the notification ID is set by the server. This suggests this is working as intended and the creation of notifications when the ID is set to a value other than none is not allowed.

My Ask:
Is there a specific reason why the error message states that a notification already exists with the ID?

This can be misleading and seems to imply that it is possible to create a notification with a custom ID however the one selected is already in use.

Possible Change:
This error message is returned directly within the create method not from a method/function called within it. I would think it would be more informative to return a message stating that the notification ID must be set to None in order to create a new notification. Unless of course there is a specific reason for this message.

[1] https://cloud.google.com/storage/docs/reporting-changes#storage_create_bucket_notifications-python
[2]

Steps to reproduce

1. Follow GCP tutorial using code snippet
2. Modify it to specify a notification ID on the notification object before it is created
3. Run the program and receive the error below.

Code example

*** Project, bucket, and topic names redacted for privacy ***

from google.cloud import storage

def create_bucket_notifications(bucket_name, topic_name, notification_id=None):

    storage_client = storage.Client('PROJECT-ID')
    bucket = storage_client.bucket(bucket_name)
    notification = bucket.notification(topic_name=topic_name, notification_id=notification_id)
    notification.create()

    print(f"Successfully created notification with ID {notification.notification_id} for bucket {bucket_name}")

create_bucket_notifications('MY-BUCKET', 'MY-TOPIC', 15)

Stack trace

*** Project, bucket, topic, and system user names redacted for privacy ***

Traceback (most recent call last):
File "/home/user/bucket-notification/text.py", line 12, in
create_bucket_notifications("MY-BUCKET", 'MY-TOPIC', 15)
File "/home/wriker/bucket-notification/text.py", line 8, in create_bucket_notifications
notification.create()
File "/usr/local/lib/python3.10/dist-packages/google/cloud/storage/notification.py", line 257, in create
raise ValueError(
ValueError: Notification already exists w/ id: 15