Merge pull request #162 from cybozu-go/errorwrap #819

Job	Run time
build (3Mi, incr-backup)	30m 40s
build (3Mi, backup-failure)	31m 7s
build (3Mi, full-backup)	31m 10s
build (200Gi, various-transfer-part-size)	25m 47s
build (3Mi, change-to-primary)	27m 15s
build (3Mi, !//)	34m 21s
build (3Mi, change-to-secondary)	25m 37s
build (10Mi, various-transfer-part-size)	25m 9s
build (3Mi, change-to-standalone)	22m 40s
	4h 13m 46s

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