fix: add nicer message for missing agent error #6475

Job	Run time
API Server unit tests	2m 32s
Agent unit tests	1m 9s
CLI unit tests	38s
WebUI unit tests	1m 51s
Build docker image	5m 50s
config-e2e	1s
Tracetesting API Server (InMemory)	4m 0s
CLI smoke tests (local)	47s
CLI smoke tests (docker)	40s
CLI e2e tests (8, 0)	1m 53s
WebUI End-to-end tests (1)	2m 52s
CLI e2e tests (8, 1)	1m 22s
Test examples (collector)	44s
WebUI End-to-end tests (2)	2m 50s
CLI e2e tests (8, 2)	1m 36s
Test examples (tracetest-jaeger)	43s
WebUI End-to-end tests (3)	2m 57s
Test examples (tracetest-opensearch)	7m 23s
Tracetesting API Server (NATS)	4m 15s
WebUI End-to-end tests (4)	2m 48s
CLI e2e tests (8, 3)	1m 31s
WebUI End-to-end tests (5)	2m 52s
CLI e2e tests (8, 4)	1m 26s
Test examples (tracetest-tempo)	41s
WebUI End-to-end tests (6)	2m 52s
CLI e2e tests (8, 5)	3m 3s
Test examples (tracetest-provisioning-env)	48s
WebUI End-to-end tests (7)	3m 46s
CLI e2e tests (8, 6)	1m 41s
WebUI End-to-end tests (8)	2m 52s
CLI e2e tests (8, 7)	3m 35s
	1h 11m 58s

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