CLEANUP: stconn: always use se_fl_set_error() to set the pending error

In mux-h2 and mux-quic we still had two places manually setting
SE_FL_ERR_PENDING or SE_FL_ERROR depending on the EOS state, instead
of using se_fl_set_error() which takes care of the condition. Better
use the specialized function for this, it will allow to centralize
the conditions. Note that this will be needed to fix a bug.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index ffbde46..a781cb0 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -6477,10 +6477,7 @@
 	    !b_data(&h2s->h2c->dbuf) &&
 	    (h2s->flags & (H2_SF_BLK_SFCTL | H2_SF_BLK_MFCTL))) {
 		TRACE_DEVEL("fctl with shutr, reporting error to app-layer", H2_EV_H2S_SEND|H2_EV_STRM_SEND|H2_EV_STRM_ERR, h2s->h2c->conn, h2s);
-		if (se_fl_test(h2s->sd, SE_FL_EOS))
-			se_fl_set(h2s->sd, SE_FL_ERROR);
-		else
-			se_fl_set(h2s->sd, SE_FL_ERR_PENDING);
+		se_fl_set_error(h2s->sd);
 	}
 
 	if (total > 0 && !(h2s->flags & H2_SF_BLK_SFCTL) &&
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 71aae71..31d5a3a 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -2394,10 +2394,7 @@
 			continue;
 
 		if (qcc->conn->flags & CO_FL_ERROR) {
-			se_fl_set(qcs->sd, SE_FL_ERR_PENDING);
-			if (se_fl_test(qcs->sd, SE_FL_EOS))
-				se_fl_set(qcs->sd, SE_FL_ERROR);
-
+			se_fl_set_error(qcs->sd);
 			qcs_alert(qcs);
 		}
 	}