Then or catch

[garyb]

Description of the change

Add thenOrCatch f g p, equivalent to p.then(f, g). Without being able to add both callback and error handler at the same step it's difficult (maybe impossible?) to handle some cases with exactly the desired semantics.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)