More output for the payload function

- The current payload silently fails when there is no copy function
- Added two warnings to explain the failure

peda.py

@@ -2859,13 +2859,15 @@ def payload_copybytes(self, target=None, data=None, template=0):
                 transfer = f
                 break
         if transfer == "":
+            warning_msg("No copy function available")
             return None
 
         headers = self.elfheader()
         start = min([v[0] for (k, v) in headers.items() if v[0] > 0])
         end = max([v[1] for (k, v) in headers.items() if v[2] != "data"])
         symbols = self.elfsymbol(transfer)
         if not symbols:
+            warning_msg("Unable to find symbols")
             return None
 
         plt_func = transfer + "_plt"