Returning errors instead of panicking

`MountInfo::new()` panics if the mountinfo file contains unexpected data.  I think it would be more appropriate to return an error instead and let the caller decide how to handle it.  [`std::io::ErrorKind::InvalidData`](https://doc.rust-lang.org/std/io/enum.ErrorKind.html#variant.InvalidData) would be a potential error variant.  What do you think?  Would you accept a PR with this change?