Fix error code received from vttablets on context expiration #954

Job	Run time
Set push variable	1s
Build and push lite (latest)	2m 14s
Build and push lite (percona80)	2m 21s
Build and push vttestserver (mysql80)	2m 9s
Build and push (bullseye, vtadmin)	1m 14s
Build and push (bullseye, vtorc)	37s
Build and push (bullseye, vtgate)	29s
Build and push (bullseye, vttablet)	38s
Build and push (bullseye, mysqlctld)	46s
Build and push (bullseye, mysqlctl)	34s
Build and push (bullseye, vtctl)	39s
Build and push (bullseye, vtctlclient)	35s
Build and push (bullseye, vtctld)	1m 12s
Build and push (bullseye, vtctldclient)	35s
Build and push (bullseye, logrotate)	24s
Build and push (bullseye, logtail)	28s
Build and push (bullseye, vtbackup)	33s
Build and push (bullseye, vtexplain)	39s
Build and push (bookworm, vtadmin)	1m 18s
Build and push (bookworm, vtorc)	38s
Build and push (bookworm, vtgate)	40s
Build and push (bookworm, vttablet)	45s
Build and push (bookworm, mysqlctld)	39s
Build and push (bookworm, mysqlctl)	37s
Build and push (bookworm, vtctl)	30s
Build and push (bookworm, vtctlclient)	46s
Build and push (bookworm, vtctld)	1m 9s
Build and push (bookworm, vtctldclient)	42s
Build and push (bookworm, logrotate)	32s
Build and push (bookworm, logtail)	28s
Build and push (bookworm, vtbackup)	38s
Build and push (bookworm, vtexplain)	37s
	26m 7s

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