test(caddy): cover invalid path branches; ci: handle go test non-zero when coverage file exists

codeql-custom-queries-go/example.ql

@@ -0,0 +1,12 @@
+/**
+ * This is an automatically generated file
+ * @name Hello world
+ * @kind problem
+ * @problem.severity warning
+ * @id go/example/hello-world
+ */
+
+import go
+
+from File f
+select f, "Hello, world!"