client: Preserve original exception chain

Author: echarrod

luno_python/base_client.py

@@ -76,7 +76,7 @@ def do(self, method, path, req=None, auth=False):
             try:
                 params = json.loads(json.dumps(req))
             except TypeError as e:
-                raise TypeError('luno: request parameters must be JSON-serializable: %s' % str(e))
+                raise TypeError('luno: request parameters must be JSON-serializable: %s' % str(e)) from e
         headers = {'User-Agent': self.make_user_agent()}
         args = dict(timeout=self.timeout, params=params, headers=headers)
         if auth: