MEDIUM: conn-stream: Use endpoint error instead of conn-stream error

Instead of relying on the conn-stream error, via CS_FL_ERR flags, we now
directly use the error at the endpoint level with the flag CS_EP_ERROR. It
should be safe to do so. But we must be careful because it is still possible
that an error is processed too early. Anyway, a conn-stream has always a
valid endpoint, maybe detached from any endpoint, but valid.
diff --git a/src/cli.c b/src/cli.c
index 35d2d36..56629b0 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1084,7 +1084,7 @@
 					}
 				break;
 			default: /* abnormal state */
-				cs->flags |= CS_FL_ERR;
+				cs->endp->flags |= CS_EP_ERROR;
 				break;
 			}