Refuse a tag entry before opening its file

A refused address left the buffer on that entry's file while contributing no
quickfix item, so a multi-selection lost its valid matches. Name the refused
tag as well, since nothing happens otherwise.

autoload/fzf/vim.vim

@@ -1684,6 +1684,7 @@ function! s:btags_sink(from, lines)
     let parts = split(line, "\t")
     let excmd = s:tag_address(parts[2])
     if empty(excmd)
+      call s:warn('Unsupported tag address: '.s:strip(parts[0]))
       continue
     endif
     call s:execute_tag_address(excmd)
@@ -1757,6 +1758,9 @@ function! s:tags_sink(from, lines)
       try
         let parts   = split(line, '\t\zs')
         let excmd   = s:tag_address(join(parts[2:-2], '')[:-2])
+        if empty(excmd)
+          throw 'Unsupported tag address: '.s:strip(parts[0])
+        endif
         let base    = fnamemodify(parts[-1], ':h')
         let relpath = parts[1][:-2]
         let abspath = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/') ? relpath : join([base, relpath], '/')
@@ -1766,9 +1770,6 @@ function! s:tags_sink(from, lines)
         else
           call s:open(expand(abspath, 1))
         endif
-        if empty(excmd)
-          continue
-        endif
         call s:execute_tag_address(excmd)
         call add(qfl, {'filename': expand('%'), 'lnum': line('.'), 'text': getline('.')})
         if empty(tagname)