wallet: Use util::Error throughout AddWalletDescriptor

32023 changed AddWalletDescriptor to return util::Error, but did not
change all of the failure cases to do so. This may result in some
callers continuing when there was actually an error. Unify all of the
failure cases to use util::Error so that all callers handle
AddWalletDescriptor errors in the same way.

The encapsulated return type is changed from ScriptPubKeyMan* to
std::reference_wrapper<DescriptorScriptPubKeyMan>. This avoids having a
value that can be interpreted as a bool, and also removes the need to
constantly dynamic_cast the returned value. The only kind of
ScriptPubKeyMan that can come out of AddWalletDescriptor is a
DescriptorScriptPubKeyMan anyways.

src/qt/test/wallettests.cpp

@@ -215,8 +215,7 @@ std::shared_ptr<CWallet> SetupDescriptorsWallet(interfaces::Node& node, TestChai
     assert(descs.size() == 1);
     auto& desc = descs.at(0);
     WalletDescriptor w_desc(std::move(desc), 0, 0, 1, 1);
-    auto spk_manager = *Assert(wallet->AddWalletDescriptor(w_desc, provider, "", false));
-    assert(spk_manager);
+    Assert(wallet->AddWalletDescriptor(w_desc, provider, "", false));
     const PKHash dest{test.coinbaseKey.GetPubKey()};
     wallet->SetAddressBook(dest, "", wallet::AddressPurpose::RECEIVE);
     wallet->SetLastBlockProcessed(105, WITH_LOCK(node.context()->chainman->GetMutex(), return node.context()->chainman->ActiveChain().Tip()->GetBlockHash()));