Handle I/O error if /proc/mdstat doesn't exist

```python
>>> from pymdstat import MdStat
>>> md = MdStat()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pymdstat/pymdstat.py", line 24, in __init__
    self.stats = self.load()
  File "pymdstat/pymdstat.py", line 81, in load
    with open(self.get_path(), 'r') as f:
FileNotFoundError: [Errno 2] No such file or directory: '/proc/mdstat'
```