[yaml] Fix examples catalog tests

[Polber]

This PR fixes a few tests in the YAML examples catalog, including enabling the Spanner tests which were previously excluded.

There are a couple of organizational changes including separating error_handling into a separate file. This was to allow LogForTesting to use the error_handling framework which is handy for examples tests that replace WriteTo transforms with LogForTesting ad-hoc that consume an error output of an upstream WriteTo transform.

This is also a prereq to #32289

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[Polber]

R: @robertwb

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