Validate block content entity in InlineBlockNormalizer::denormalize

Problem/Motivation

At present InlineBlockNormalizer doesn't validate the denormalized inline block. It should and throw an error if it doesn't validate.

Steps to reproduce

Put a layout with an invalid field value for an inline block, save the layout and see the block is allowed to be saved even though invalid

Proposed resolution

Call validate on the entity before serializing. Throw an error if invalid

Remaining tasks

Implementation
Tests

User interface changes

API changes

Data model changes