Exit with error code on yarn-install failure (#292)

Summary:
If installation fails, we should exit with an error code and not just a
message. This should help narrow down the CI failures, too.
Pull Request resolved: https://github.com/facebook/flipper/pull/292

Reviewed By: danielbuechele

Differential Revision: D10255759

Pulled By: passy

fbshipit-source-id: a6b033deadf11069e6c5665870a811ae2d0ec5ef

scripts/yarn-install.js

@@ -54,4 +54,5 @@ Promise.all(
   .catch(err => {
     console.error('❌  Installing plugin dependencies failed.');
     console.error(err);
+    process.exit(1);
   });