Use the ErrorWithStatus+try & except pattern for layer errors

[ca-scribner]

_update_layer() may catch a ChangeError emitted when doing a container.add_layer(). In _update_layer() we directly set the unit's status to Blocked based on this. This is different from our usual pattern of, when a helper has an opinion about a status to set, emitting an ErrorWithStatus which suggests the status of the charm.

We should also think about the type of status set here. Should it be Blocked, or Error?

See [this conversation[(https://github.com//pull/82#discussion_r1130979299) for more details