store: reword panic message and add test to check httputil client has… #283

Job	Run time
snap-builds	0s
cache-build-deps	1m 59s
static-checks (1.18)	4m 54s
static-checks (latest/stable)	8m 17s
unit-tests (1.18, normal)	9m 10s
unit-tests (1.18, snapd_debug)	7m 12s
unit-tests (1.18, withbootassetstesting)	7m 2s
unit-tests (1.18, nosecboot)	9m 31s
unit-tests (1.18, faultinject)	9m 17s
unit-tests (latest/stable, normal)	7m 48s
unit-tests (latest/stable, snapd_debug)	9m 31s
unit-tests (latest/stable, withbootassetstesting)	7m 40s
unit-tests (latest/stable, nosecboot)	7m 58s
unit-tests (latest/stable, faultinject)	7m 40s
code-coverage	8s
spread-nested	0s
spread	0s
	1h 38m 7s

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