Say why CTRL-O did nothing on a refused address

The sinks name the tag; the Show callback returned without a word, so the key
looked broken.

autoload/fzf/vim.vim

@@ -776,7 +776,10 @@ function! s:goto_entry(winid, bufnr, dir, kind, entry) abort
     let path = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/')
           \ ? relpath : join([fnamemodify(parts[-1], ':h'), relpath], '/')
     let path = expand(path, 1)
-    if empty(excmd) || !filereadable(path)
+    if empty(excmd)
+      return s:warn('Unsupported tag address: '.s:strip(parts[0]))
+    endif
+    if !filereadable(path)
       return
     endif
     let cmds = s:edit_cmds(a:winid, path)
@@ -788,7 +791,7 @@ function! s:goto_entry(winid, bufnr, dir, kind, entry) abort
     endif
     let excmd = s:tag_address(parts[2], 0)
     if empty(excmd)
-      return
+      return s:warn('Unsupported tag address: '.s:strip(parts[0]))
     endif
     let cmds = ['keepalt keepjumps hide buffer '.a:bufnr,
           \ s:sandboxed('keepjumps '.excmd), 'normal! zvzz']