[File] Python bindings to libmagic

[Hoël Bézier]

Hi,

the libmagic python bindings seem to have very limited error handling 
capabilities, mostly returning None instead of the expected object when failing 
and never raising any Exception of any kind.

This makes proper error handling a bit harder on the user side, and to make 
sure I do things properly I’d like to clarify something.

The `open` routine pretends it returns None on failure, but it is defined in 
the following way:

```
def open(flags):
     """
     Returns a magic object on success and None on failure.
     Flags argument as for setflags.
     """
     return Magic(_open(flags))
```

with the Magic constructor being:

```
class Magic(object):
     def __init__(self, ms):
         self._magic_t = ms
```

So it seems to me that the `open` routine always returns a Magic object and 
cannot fail. The `_open` routine on the other hand might fail, and thus the 
returned Magic object would have a _magic_t attribute set to None.

If I understood things properly, maybe the documentation would need to be 
updated so that users may handle failures properly.

Regards,
Hoël
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