bug#18067: 24.3.92; python-completion-complete-at-point signals an error when there's no python-shell buffer

[Dmitry]

1. Open a .py file (and nothing else)

2. Press C-M-i, see the error.

I don't think it should signal the error (or even show a message) when
it can't perform completion, because that blocks any other elements in
completion-at-point-functions that may go after it (such as
tags-completion-at-point-function).

Originally reported here:
https://github.com/bbatsov/prelude/issues/610#issuecomment-49455235

In GNU Emacs 24.3.92.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-07-11 on axl
Repository revision: 117369 rgm@gnu.org-20140710180904-p1ng9uu1zmkyaxju
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04 LTS