[dhcp_relay] Update error log for dhcp server route incorrect (#15898)

Error log for default route incorrect is ambiguous

tests/dhcp_relay/conftest.py

@@ -147,7 +147,9 @@ def validate_dut_routes_exist(duthosts, rand_one_dut_hostname, dut_dhcp_relay_da
     """Fixture to valid a route to each DHCP server exist
     """
     py_assert(wait_until(120, 5, 0, check_routes_to_dhcp_server, duthosts[rand_one_dut_hostname],
-                         dut_dhcp_relay_data), "Failed to find route for DHCP server")
+                         dut_dhcp_relay_data),
+              "Packets relayed to DHCP server should go through default route via upstream neighbor, but now it's" +
+              " going through mgmt interface, which means device is in an unhealthy status")
 
 
 @pytest.fixture(scope="module")