don't kill snippet buffer unless saved

Split the quit-window part of yas-load-snippet-buffer into
yas-load-snippet-buffer-and-close, and only quit if the snippet was
saved.

This resolves #417, where you could lose a snippet by giving the wrong
answer in a prompt.
move snippet saving from loading to closing

yas-load-snippet-buffer now just loads the snippet,
yas-load-snippet-buffer-and-close also saves it.
let C-c C-c save existing snippets too
Merge pull request #423 from npostavs/load-snippet-ui

Improve load snippet ui