Fix merge_each error propagation

[fallintoplace]

Summary

• preserve an existing error when nested work also signals a break
• complete normally after the delayed error item is consumed so sequence consumers can publish it
• cover errors from nested value senders, nested sequences, and the outer sequence

Testing

• ctest --test-dir build --output-on-failure -j 8
• full test.exec run with -fno-exceptions

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[ericniebler]

/ok to test 841c8fe

[ericniebler]

/ok to test 188225e