wallet: use PSBTError::EXTERNAL_SIGNER_NOT_FOUND

Instead of throwing a runtime error, let the caller decide how to handle a missing signer.

GUI code was already in place to handle this, but it was unused until this commit.

Fixes #32426

Additionally use LogWarning instead of std::cerr.

Author: Sjors

src/wallet/external_signer_scriptpubkeyman.cpp

@@ -94,11 +94,14 @@ std::optional<PSBTError> ExternalSignerScriptPubKeyMan::FillPSBT(PartiallySigned
     if (complete) return {};
 
     auto signer{GetExternalSigner()};
-    if (!signer) throw std::runtime_error(util::ErrorString(signer).original);
+    if (!signer) {
+        LogWarning("%s", util::ErrorString(signer).original);
+        return PSBTError::EXTERNAL_SIGNER_NOT_FOUND;
+    }
 
     std::string failure_reason;
     if(!signer->SignTransaction(psbt, failure_reason)) {
-        tfm::format(std::cerr, "Failed to sign: %s\n", failure_reason);
+        LogWarning("Failed to sign: %s\n", failure_reason);
         return PSBTError::EXTERNAL_SIGNER_FAILED;
     }
     if (finalize) FinalizePSBT(psbt); // This won't work in a multisig setup