Show the names of missing plugins

Ref wklken#366

Author: liuchengxu

core/autoload/spacevim/vim/plug.vim

@@ -6,8 +6,11 @@ endfunction
 
 " https://github.com/junegunn/vim-plug/wiki/extra#automatically-install-missing-plugins-on-startup
 function! spacevim#vim#plug#check(...) abort
-  let l:msg = '[space-vim] Some layers need to install the missing plugins.'
-  if len(filter(values(g:plugs), '!isdirectory(v:val.dir)'))
+  let l:msg = '[space-vim] Need to install the missing plugins: '
+  let missing = filter(values(g:plugs), '!isdirectory(v:val.dir)')
+  if len(missing)
+    let plugs = map(missing, 'split(v:val.dir, "/")[-1]')
+    let l:msg .= string(plugs)
     if a:0 == 1
       if s:ask(l:msg)
         silent PlugInstall --sync | q