Puma does not handle SyntaxError properly

Please take a look at the responses for these two Rack apps. It seems Puma doesn't handle SyntaxError properly as it does for regular errors:

``` ruby
# config.ru for regular error
run ->(env) { raise 'Error' }

# config.ru for SyntaxError
run ->(env) { require_relative 'abc' }

# abc.rb
class # invalid syntax
```

Would it be possible for Puma to fix the HTTP response in the latter case?
