The error code PSBTError::EXTERNAL_SIGNER_NOT_FOUND is never returned anywhere in our codebase, hence code that handles it (currently done only in the GUI) is effectively dead. I have never looked deeper into external signing, but at a first glance the two obvious logical choices would be to either:

• simply get rid of this error code and its handling (if e.g. a more generic error thrown in this case already provides enough information to the user)
• implement the returning of the error condition; the areas that need to be touched are likely ExternalSignerScriptPubKeyMan::GetExternalSigner and its call-site in ::FillPSBT below

(Noticed while reviewing #31622, where new PSBTError codes are added in the third commit)