[MINOR][HOTFIX] Fix python code style issue

## What changes were proposed in this pull request?

Travis complains "E722 do not use bare except" in python-client style check. Minor changes to fix it.

Author: jerryshao <sshao@hortonworks.com>

Closes #60 from jerryshao/python-style-fix.

python-api/src/main/python/livy/client.py

@@ -335,7 +335,7 @@ class HttpClient(object):
                         headers = {'X-Requested-By': 'livy'}
                         self._conn.send_request("DELETE", session_uri,
                             headers=headers)
-                except:
+                except Exception:
                     raise Exception(traceback.format_exc())
                 self._stopped = True