Switch to a proper way to pop up warnings as errors

Author: MoOx

bsconfig.json

@@ -16,6 +16,9 @@
     }
   ],
   "namespace": false,
-  "bsc-flags": ["-bs-no-version-header", "-warn-error @a"],
+  "bsc-flags": ["-bs-no-version-header"],
+  "warnings": {
+    "error": true
+  },
   "bs-dependencies": ["reason-react", "reason-react-native"]
 }