[RFC] vim-patch:7.4.337

Problem:    When there is an error preparing to edit the command line, the
            command won't be executed. (Hirohito Higashi)
Solution:   Reset did_emsg before editing.

https://code.google.com/p/vim/source/detail?r=v7-4-337

This is a very simple patch w/o any changes (apart from the comment style).