feat: Handle blocking content errors

[lornakelly]

Description

Add UI error state for cases where the workflow cannot be rendered. When parsing fails in a way that prevents building the diagram, display error feedback instead of attempting to render.

Motivation

Avoid broken or empty diagrams by clearly communicating when rendering is not possible.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.