TypeError: 'bool' object is not subscriptable when calling name commands if too many payment requests are active

[JeremyRand]

Steps to reproduce

1. Run name_autoregister in a loop for circa 20-30 names, before any of the registration transactions are broadcasted.

Expected result

No errors.

Observed result

The command will raise TypeError: 'bool' object is not subscriptable once there are too many payment requests to create a new request.

Analysis

1. We should accept a force parameter for all commands that can (transitively) call the add_request command, this parameter would get passed through to add_request.
2. We should furthermore expose this parameter to the GUI, similarly to what create_bitcoin_request in main_window.py does.
3. Finally, we should edit the GUI text so that it makes clear that waiting for queued transactions (if any exist) to be broadcasted will resolve the issue.
4. Some of the commits that aren't in master yet (perhaps the auction commands?) may also be affected; we should audit for that before closing this issue.