MultiJson load error should match

"Recently" `MultiJson.decode` was deprecated in favor or `#load` (and `#encode` with `#dump`). I think this is more semantic.

But the resulting errors still reference the "old" methods. For example:

```ruby
MultiJson.load("invalid json") #=> MultiJson::DecodeError
```

This should really be a `LoadError`, IMO.

It's a little confusing that I'm loading content, but rescuing a decode error. Thoughts?