check if data folder is a writable directory

[stefan0xC]

Adds a check if the data folder is a directory and also returns a more verbose error message if the permission was denied when creating a file.

I tried adding something like the following to check_data_folder() as well

let metadata = path.metadata().unwrap();
if metadata.permissions().readonly() {
    warn!("Data folder '{}' is read-only.", data_folder);
}

However, this does not really work because readonly() only checks if anyone has write permissions. 😖