Fix error message when volt cannot overwrite vimrc #222
Description
Got
$ volt build
[INFO] Full building /home/tyru/.vim/pack/volt/opt directory ...
[INFO] Installing vimrc and gvimrc ...
[ERROR] Failed to build: '/home/tyru/.vim/vimrc' does not have magic comment
Expected
$ volt build
[ERROR] Failed to build: '/home/tyru/.vim/vimrc' is not a auto-generated file. please move to '/home/tyru/volt/rc/default/vimrc' and re-run 'volt build'