feat: better errors in promise finally

Co-authored-by: Copilot <[email protected]>

Author: sudo-tee

lua/opencode/promise.lua

@@ -193,11 +193,11 @@ function Promise:finally(callback)
   local new_promise = Promise.new()
 
   local handle_finally = function()
-    local ok, _ = pcall(callback)
+    local ok, err = pcall(callback)
     -- Ignore callback errors and result, finally doesn't change the promise chain
     if not ok then
       -- Log error but don't propagate it
-      vim.notify('Error in finally callback', vim.log.levels.WARN)
+      vim.notify('Error in finally callback: ' .. tostring(err), vim.log.levels.WARN)
     end
   end