Catch wiring semantics

[cefn]

Decide whether catch handler should be wired with the semantic of promise.then().catch() or promise.catch()

At

watchable/packages/unpromise/src/unpromise.ts

Line 108 in bb7131e

if ("catch" in thenReturn) {

the catch handler for an Unpromise is wired to the catch() returned by then() rather than wiring directly to this.promise.catch()

This is harmless given the then handler chained beforehand (which is synchronous and guaranteed to execute without error) but I recall this was a deliberate design decision from the semantics of promises, but would be good to articulate the reason, or reconsider the choice.