vimUtils.buildVimPlugin: write vim errors to stderr

Previously vim would silently fail when help tags couldn't be generated.
We need to pass the "verbose" flag (with verbose level 1) to convince
vim to print errors to standard error.

pkgs/misc/vim-plugins/build-vim-plugin.nix

···
         # build help tags
         if [ -d "$target/doc" ]; then
           echo "Building help tags"
-          if ! ${vim}/bin/vim -N -u NONE -i NONE -n -E -s -c "helptags $target/doc" +quit!; then
+          if ! ${vim}/bin/vim -N -u NONE -i NONE -n -E -s -V1 -c "helptags $target/doc" +quit!; then
             echo "Failed to build help tags!"
             exit 1
           fi