refactor: more descriptive "not a conform" message #1104

Job	Run time
build / build	12s
run (oidcc-client-basic-certification-test-plan)	1m 51s
run (oidcc-client-hybrid-certification-test-plan)	2m 2s
run (oidcc-client-test-plan)	1m 56s
run (oidcc-client-test-plan, code id_token)	2m 29s
run (fapi1-advanced-final-client-test-plan, private_key_jwt)	2m 51s
run (fapi1-advanced-final-client-test-plan, mtls)	2m 19s
run (fapi1-advanced-final-client-test-plan, plain_response, by_value)	2m 57s
run (fapi2-security-profile-id2-client-test-plan, private_key_jwt, dpop)	2m 33s
run (fapi2-security-profile-id2-client-test-plan, private_key_jwt, mtls)	2m 11s
run (fapi2-security-profile-id2-client-test-plan, mtls, dpop)	2m 23s
run (fapi2-security-profile-id2-client-test-plan, mtls, mtls)	2m 17s
run (fapi2-message-signing-id1-client-test-plan)	3m 6s
run (fapi2-message-signing-id1-client-test-plan, plain_oauth)	2m 8s
	31m 15s

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