vim-patch:7.4.275

Problem:  When changing the type of a sign that hasn't been
          placed ther is no error message.
Solution: Add an error message. (Christian Brabandt)

Author: Bram Moolenaar

https://code.google.com/p/vim/source/detail?r=8a3117a4887c1e12a1165c9719491f96753
Merge #738 'vim-patch:7.4.275'