Add accessors for ChildWorkflowExecutionError fields #2312

Job	Run time
build-and-test (ubuntu-latest, oldstable)	17m 24s
cloud-test	0s
build-and-test (ubuntu-latest, stable)	26m 25s
build-and-test (macos-intel, oldstable)	19m 37s
build-and-test (macos-intel, stable)	17m 56s
build-and-test (macos-arm, oldstable)	17m 36s
build-and-test (macos-arm, stable)	17m 32s
build-and-test (windows-latest, oldstable)	21m 44s
build-and-test (windows-latest, stable)	20m 56s
features-test / test	2m 17s
	2h 41m 27s

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