changed function error code for external function processing (#101)

Co-authored-by: Lorenzo Curcio <lorenzocurcio@gmail.com>

Author: lor1113

elementpath/xpath_tokens/functions.py

@@ -413,12 +413,12 @@ def evaluate(self, context: ta.ContextType = None) -> Any:
             for k, (arg, st) in enumerate(zip(args, self.sequence_types), start=1):
                 if not match_sequence_type(arg, st, self.parser):
                     msg = f"{ordinal(k)} argument does not match sequence type {st!r}"
-                    raise self.error('XPDY0050', msg)
+                    raise self.error('XPTY0004', msg)
 
             result = self.callback(*args)
             if not match_sequence_type(result, self.sequence_types[-1], self.parser):
                 msg = f"Result does not match sequence type {self.sequence_types[-1]!r}"
-                raise self.error('XPDY0050', msg)
+                raise self.error('XPTY0004', msg)
             return result
 
         return self.callback(*args)