Let the original LoadError exception surface

The proposed solution is the bare minimum, (IMO) better options are:

- use a custom exception that inherits from ArgumentError catching only exceptions that are local to the method or explicitly raised in `#load_adapter_from_string_name` (using `#const_defined?`?)
- prepend/append a "helper" message to the actual exception raise
- remove the rescue completely and let the originals reach the user

### Why?

I had a badly compiled json gem and gone mad because the message was pointing me in the wrong direction. Worst than that `oauth2` was [obliterating the exception](https://github.com/intridea/oauth2/blob/master/lib/oauth2/response.rb#L53) completely.