[Standard] Exception are incorrectly handled when RawDataOuput is used

# Description
When a request use RawDataOutput and the handler function generate an exception, the result is an empty response.

The OGC standard state:

> In the most primitive case, when a response form of "RawDataOutput" is requested,
> process execution is _successful_, and only one complex output is produced, then the
> Execute operation response will consist simply of that one complex output in its raw form
> returned directly to the client. For example, if in the case where a WPS process creates
> one GIF image as its output, that GIF image would be returned to the client as a direct
>response to the Execute request.
> In all other cases, the response to a valid Execute operation request is an
> ExecuteResponse XML document.

As far as I understand, if a RawDataOutput request is unsuccessful, such as raising an Exception, the result should be and ExecuteResponse XML.

# Environment

- operating system:
Linux 4.X
- Python version:
3.5.4 - probably all
- PyWPS version:
github develop branch
- source/distribution
- [x] git clone
- web server
- [x] Apache/mod_wsgi

# Steps to Reproduce
Create a WPS, raise a Exception in _handler and make a request to that service with RawDataOutput
# Additional Information