Name a too-short Tags entry the way BTags does

s:strip(parts[0]) on an empty line raises E684, which the loop then reports in
place of the message.

autoload/fzf/vim.vim

@@ -1798,7 +1798,7 @@ function! s:tags_sink(from, lines)
         let parts   = split(line, '\t\zs')
         let excmd   = s:tag_address(join(parts[2:-2], '')[:-2], 1)
         if empty(excmd)
-          throw 'Unsupported tag address: '.s:strip(parts[0])
+          throw 'Unsupported tag address: '.s:strip(get(parts, 0, line))
         endif
         let base    = fnamemodify(parts[-1], ':h')
         let relpath = parts[1][:-2]