linting: error as default serverity

Ensure that CI is able to pick liniting violation until tslint matures
further: palantir/tslint#2604
This makes the output look slightly different but it contains the same
information.

tslint.json

@@ -1,5 +1,5 @@
 {
-  "defaultSeverity": "warning",
+  "defaultSeverity": "error",
   "extends": [
     "tslint:recommended",
     "./tslint-wikimedia-eslint-rules-port.json"