Update client and fix locking

[justvanrossum]

• Sync client.py with upstream: this will cause an HTTPError to be raised for any failing api request
• Ensure that the server is in control about whether the lock is granted
• Keep a flag so we can know whether we acquired the lock or not
• Show a dialog if we open the window but couldn't acquire the lock
• Do not attempt to save if we do not hold the lock

The dialog: