mutable/retrieve.py: inline the single-use function _remove_reader.

A simple refactoring. Doesn't even require a new or updated unit test.

Author: Zooko O'Whielacronx <zooko@zooko.com>
Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Raise NotEnoughSharesError if there are initially not enough shares for a mutable retrieve. refs #1742

We also change the order of setting up attributes in Retrieve so that _raise_notenoughshareserror() can be called from _setup_download.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
mutable/retrieve.py: improve comments. refs #1742

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Include the number of remaining shares of the right version in NotEnoughSharesError messages.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Add a test for raising NotEnoughSharesError when a Retrieve object is created with an insufficient servermap. refs #1742

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>