[validation] Add detailed txin/txout information for script error messages

Don't just report which script error occurred, but which in which input of which transaction,
and which UTXO was being spent.

src/validation.h

@@ -347,7 +347,7 @@ public:
     CScriptCheck(CScriptCheck&&) = default;
     CScriptCheck& operator=(CScriptCheck&&) = default;
 
-    std::optional<ScriptError> operator()();
+    std::optional<std::pair<ScriptError, std::string>> operator()();
 };
 
 // CScriptCheck is used a lot in std::vector, make sure that's efficient