Exit with non-zero exit code from yarn mintable. (#51)

Thanks @blimmer – On failure, catch rejected promises and exit with a non-zero exit code
to break the build.
Relates to #43

src/scripts/mintable.js

@@ -186,4 +186,7 @@
     default:
       break
   }
-})()
+})().catch((e) => {
+  console.error(e);
+  process.exit(1);
+})