Fixed storage container creating existing directory

Made the storage container code check if the directory exists before
creating a new one over the top.

The previous behaviour was causing a crash on software created using the
PSM build (and perhaps others) of MonoGame.

Refer to issue [**#3061**](https://github.com/mono/MonoGame/issues/3061) for more information.
The build is failing for Windows8....
```
Storage\StorageContainer.cs(139,18): error CS0103: The name 'Directory' does not exist in the current context
```
I'm unsure if the 'Directory' class is supported on Windows 8 Store apps or if it changed namespaces or what.

It might be best to just wrap your change here with #if statements:

```
#if !WINDOWS_STOREAPP
if (!Directory.Exists(_storagePath))
#endif
```