bump_fee: fix error message

gui/qt/main_window.py

@@ -2633,7 +2633,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         try:
             new_tx = self.wallet.bump_fee(tx, delta)
         except BaseException as e:
-            self.show_error(e)
+            self.show_error(str(e))
             return
         if is_final:
             new_tx.set_sequence(0xffffffff)

lib/wallet.py

@@ -1020,7 +1020,7 @@ class Abstract_Wallet(PrintError):
                 outputs[i] = otype, address, value - delta
                 break
         else:
-            raise BaseException("cannot bump fee")
+            raise BaseException("cannot bump fee: could not find a change output")
         new_tx = Transaction.from_io(inputs, outputs)
         return new_tx