Syntax error on curtin/block/__init__.py#L317-L318

[Cho Jaeho]

There is a syntax error on 'curtin/block/__init__.py#L317-L318'
- https://github.com/canonical/curtin/blob/master/curtin/block/__init__.py#L317-L318

Python string interpolation is not working because of omitting of parenthesis. As a result, it prints "TypeError: not enough arguments for format string". but that is not what I expected. It should print RuntimeError for "Device ~ not present in sfdisk dump ~".

Below is original one:
```
        raise RuntimeError('Device %s not present in sfdisk dump:\n%s' %
                           devpath, util.json_dumps(sfdisk_info))
```

It should be like this:
```
        raise RuntimeError('Device %s not present in sfdisk dump:\n%s' %
                           (devpath, util.json_dumps(sfdisk_info)))
```