[Lib][Qt] Pulled some minor tweaks from upstream to wallet storage

Deleting a wallet from outside the app while it's running could cause
weirdness (see: spesmilo#4984 and
spesmilo#5082). Follow ups to fix
that.

gui/qt/main_window.py

@@ -2677,11 +2677,13 @@ def remove_wallet(self):
     def _delete_wallet(self, password):
         wallet_path = self.wallet.storage.path
         basename = os.path.basename(wallet_path)
-        self.gui_object.daemon.stop_wallet(wallet_path)
+        r = self.gui_object.daemon.delete_wallet(wallet_path)  # implicitly also calls stop_wallet
         self.close()
-        os.unlink(wallet_path)
         self.update_recently_visited(wallet_path) # this ensures it's deleted from the menu
-        self.show_error("Wallet removed:" + basename)
+        if r:
+            self.show_error(_("Wallet removed: {}").format(basename))
+        else:
+            self.show_error(_("Wallet file not found: {}").format(basename))
 
     @protected
     def show_seed_dialog(self, password):

lib/daemon.py

@@ -245,6 +245,13 @@ def add_wallet(self, wallet):
     def get_wallet(self, path):
         return self.wallets.get(path)
 
+    def delete_wallet(self, path):
+        self.stop_wallet(path)
+        if os.path.exists(path):
+            os.unlink(path)
+            return True
+        return False
+
     def stop_wallet(self, path):
         # Issue #659 wallet may already be stopped.
         if path in self.wallets:

lib/storage.py

@@ -196,6 +196,9 @@ def _write(self):
             os.fsync(f.fileno())
 
         mode = os.stat(self.path).st_mode if self.file_exists() else stat.S_IREAD | stat.S_IWRITE
+        if not self.file_exists():
+            # See: https://github.com/spesmilo/electrum/issues/5082
+            assert not os.path.exists(self.path)
         # perform atomic write on POSIX systems
         try:
             os.rename(temp_path, self.path)