add yarn-error.log to gitignore

Summary:
Due to react-native-cli use yarn if yarn is available, we need to add yarn-error.log to .gitignore.
Closes https://github.com/facebook/react-native/pull/11583

Differential Revision: D4359507

fbshipit-source-id: 0c7582215a31e0d03dced138c8275225910713ee

local-cli/templates/HelloWorld/_gitignore

@@ -34,6 +34,7 @@ local.properties
 #
 node_modules/
 npm-debug.log
+yarn-error.log
 
 # BUCK
 buck-out/