user-friendly error message when run/start in a session (#54) #198

Job	Run time
build-and-test (debug, none)	5m 37s
build-and-test (debug, address)	5m 19s
build-and-test (debug, leak)	4m 50s
build-and-test (debug, undefined)	5m 46s
build-and-test (debug, thread)	4m 42s
build-and-test (release, none)	6m 27s
build-and-test (release, leak)	4m 59s
build-and-test (release, undefined)	13m 49s
build-and-test (release, thread)	5m 43s
	57m 12s

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