properly exit neovim headless with <C-c>

lua/fzf-lua/libuv.lua

@@ -326,7 +326,9 @@ M.spawn_stdio = function(opts, fn_transform, fn_preprocess)
         -- <C-c> err will be either EPIPE or ECANCELED
         -- don't really need to do anything since the
         -- processs will be killed anyways with os.exit()
-        if err then io.stderr:write("pipe:write Error: "..err) end
+        if err then
+          io.stderr:write(("pipe:write error: %s\n"):format(err))
+        end
         if cb then cb(err) end
       end)
   end
@@ -350,8 +352,16 @@ M.spawn_stdio = function(opts, fn_transform, fn_preprocess)
       if stdout then
         pipe_write(stdout, data, cb)
       else
-        io.stdout:write(data)
-        cb(nil)
+        -- on success: rc=true, err=nil
+        -- on failure: rc=nil, err="Broken pipe"
+        -- cb with an err ends the process
+        local rc, err = io.stdout:write(data)
+        if not rc then
+          io.stderr:write(("io.stdout:write error: %s\n"):format(err))
+          cb(err or true)
+        else
+          cb(nil)
+        end
       end
     end