[FIX] web: display better tracebacks in debug=assets

A previous commit (934e0896160272542dd00) modified QUnit to make sure we
can display better tracebacks in debug=assets mode.  However, this
commit did not apply to errors coming from failing promises.

To fix this, we can simply keep a reference to the error in that
specific case.

closes odoo/odoo#67048

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>

addons/web/static/lib/qunit/qunit-2.9.1.js

@@ -3376,6 +3376,7 @@
   					then.call(promise, function () {
   						resume();
   					}, function (error) {
+  						lastError = error;
   						message = "Promise rejected " + (!phase ? "during" : phase.replace(/Each$/, "")) + " \"" + test.testName + "\": " + (error && error.message || error);
   						test.pushFailure(message, extractStacktrace(error, 0));