Form binding should keep current errors

The bind method currently resets form errors. This makes it hard or even impossible to chain form validation. For instance rejecting data if a multipart file is missing cannot be achieved like this:

```scala
form.withGlobalError("file.missing").bindFromRequest.fold(
  errors,
  ok
)
```
  
Therefor:
* Present errors should be retained when binding data to a form
* fold should not only check the presence of a value, but also whether there are errors or not