Check for S3 write permission and show warning if can't write

Resolves #124 

Checking the permissions of a user doesn't seem easy within the AWS SDK ([related](https://github.com/aws/aws-sdk-php/issues/121)), so went with this implementation.
Instead of nesting a big block like this, I always like to return early:

```php
if ( false !== ( $transient = get_transient( $transient_key ) ) ) {
    return $transient;
}
```