wallet, logging: Switch LogInfo to LogError in AddWalletDescriptor

Seems like all of these cases are treated as errors, so it makes sense to use
the corresponding log level.

Change suggested by hodlinator in bitcoin#30343 (comment)

Author: ryanofsky

src/wallet/wallet.cpp

@@ -4166,7 +4166,7 @@ util::Result<std::reference_wrapper<DescriptorScriptPubKeyMan>> CWallet::AddWall
 
     auto spk_man = GetDescriptorScriptPubKeyMan(desc);
     if (spk_man) {
-        LogInfo(m_log, "Update existing descriptor: %s\n", desc.descriptor->ToString());
+        LogError(m_log, "Update existing descriptor: %s\n", desc.descriptor->ToString());
         if (auto spkm_res = spk_man->UpdateWalletDescriptor(desc); !spkm_res) {
             return util::Error{util::ErrorString(spkm_res)};
         }