No repr or str on some exceptions

[Lucidiot]

apistar.exceptions.ErrorResponse and ClientError do not have __repr__ or __str__ implemented; this causes error messages to be empty in tracebacks and makes debugging harder. Here is what I get with IPython for example:

~/Prog/apistar/apistar/client/transports.py in send(self, method, url, query_params, content, encoding)
     76             title = "%d %s" % (response.status_code, response.reason)
     77             raise exceptions.ErrorResponse(
---> 78                 title=title, status_code=response.status_code, content=result
     79             )
     80 

ErrorResponse: 

The exception actually had a name, status code and content set.