Phrase creation and deletion error handling.

[simjanos-dev]

• [ ] Users should not be able to create or delete a phrase before the previous delete or create phrase request returns.
• [ ] There should be an error message if creating a phrase fails.
• [ ] There should be an error message if deleting a phrase fails.

(Phrase ID is returned by the create phrase request, so it is impossible to delete a phrase that was just created and the create request has not returned the ID yet.)