Fix [make promote] to terminate with error code 0.

Makefile

@@ -147,7 +147,7 @@ dune:
 
 .PHONY: promote
 promote:
-	@ dune build @test --auto-promote --display short >/dev/null 2>&1
+	@ dune build @test --auto-promote >/dev/null 2>&1 || true
 
 # [make depend] regenerates the files dune.auto. This command should
 # be run every time some tests are added or removed or renamed in the