dlload throws LoadError instead of Fiddle::DLError

### Environment

```
jruby 9.2.8.0 (2.5.3) 2019-08-12 a1ac7ff Java HotSpot(TM) 64-Bit Server VM 25.192-b12 on 1.8.0_192-b12 +jit [darwin-x86_64]
```

### Expected Behavior

```ruby
require "fiddle/import"

module Hello
  extend Fiddle::Importer
  dlload "hi"
end
```

raises `Fiddle::DLError` like MRI

### Actual Behavior

Raises `LoadError`