fix(vim.pack): add() stops unexpectedly on package load error #34787

Problem:
Error when adding a plugin will make all following plugins not
`:packadd`ed

Solution:
- add() should handle errors from :packadd with pcall()

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/plugin/pack_spec.lua

@@ -4,6 +4,11 @@ describe('vim.pack', function()
       -- TODO
     end)
 
+    pending('reports errors after loading', function()
+      -- TODO
+      -- Should handle (not let it terminate the function) and report errors from pack_add()
+    end)
+
     pending('respects after/', function()
       -- TODO
       -- Should source 'after/plugin/' directory (even nested files) after