Added HTTP error handling in _get_body (#38)

* Update client.py

Modified behaviour of _get_body in absence of a body to get: 
in such cases, there is actually a HTTP error which will be raised

Author: vlavorini

openrouteservice/client.py

@@ -225,10 +225,14 @@ def req(self):
     @staticmethod
     def _get_body(response):
         """Returns the body of a response object, raises status code exceptions if necessary."""
-        body = response.json()
+        try:
+            body = response.json()
+        except json.JSONDecodeError:
+            raise exceptions.HTTPError(response.status_code)
+
         # error = body.get('error')
         status_code = response.status_code
-
+        
         if status_code == 429:
             raise exceptions._OverQueryLimit(
                 status_code,