Fix file deletion in error cases.
Fix file deletion in error cases.

We want to use `fs.unlinkAsync`, because that is the one that returns a promise
so that we can call `then` from the return value.

thaliproject/Thali_CordovaPlugin,thaliproject/Thali_CordovaPlugin,thaliproject/Thali_CordovaPlugin,thaliproject/Thali_CordovaPlugin,thaliproject/Thali_CordovaPlugin