Clarify FallbackError exception handling and error codes in fallback matrix documentation

[JohnnyWilson16]

Context

As outlined in the Contributor Roadmap, documentation should clearly explain how out-of-core execution errors behave when users configure strict execution policies.

Task

Expand docs/fallback-matrix.md with explicit Python code examples showing how fd.FallbackError is raised when fallback_policy="error" is triggered, and how users can programmatically catch and inspect the exception attributes.

Acceptance Criteria

• Update docs/fallback-matrix.md under the fallback_policy="error" section.
• Add an explicit Python code example using try ... except fd.FallbackError as err: demonstrating how to inspect exception properties (e.g. err.trigger, err.backend).
• Show the exact error message and recommended native alternative escape hatches.
• Verify documentation builds without warnings using mkdocs build --strict.

Pointers

• Target documentation file: docs/fallback-matrix.md
• Exception implementation: search for class FallbackError in src/freshdata/

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