Offer more explicit errors than "permission denied"

Today if GAM can't open or write to a file it returns an error like:

```
ERROR: File: <path>  [Errno 13] : Permission denied <path>
```

that's not an awful error but we might be able to offer a bit more detail like:

```
ERROR: GAM cannot write to file <path>. Please ensure your user has permissions to write to this file on your filesystem.
```

or similar.