network: don't save negative ETA fee estimates

-1 means bitcoind could not give an estimate

electrum/gui/qt/main_window.py

@@ -1542,8 +1542,8 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
             tx = self.wallet.make_unsigned_transaction(
                 coins, outputs, self.config, fixed_fee=fee_estimator,
                 is_sweep=is_sweep)
-        except NotEnoughFunds:
+        except (NotEnoughFunds, NoDynamicFeeEstimates) as e:
-            self.show_message(_("Insufficient funds"))
+            self.show_message(str(e))
             return
         except BaseException as e:
             traceback.print_exc(file=sys.stdout)

electrum/network.py

@@ -357,8 +357,9 @@ class Network(PrintError):
         self.notify('fee_histogram')
         for i, task in fee_tasks:
             fee = int(task.result() * COIN)
-            self.config.update_fee_estimates(i, fee)
             self.print_error("fee_estimates[%d]" % i, fee)
+            if fee < 0: continue
+            self.config.update_fee_estimates(i, fee)
         self.notify('fee')
 
     def get_status_value(self, key):

electrum/util.py

@@ -77,7 +77,9 @@ def base_unit_name_to_decimal_point(unit_name: str) -> int:
         raise UnknownBaseUnit(unit_name) from None
 
 
-class NotEnoughFunds(Exception): pass
+class NotEnoughFunds(Exception):
+    def __str__(self):
+        return _("Insufficient funds")
 
 
 class NoDynamicFeeEstimates(Exception):