config: reject non-json-serialisable writes

see #4788

electrum/simple_config.py

@@ -140,6 +140,12 @@ class SimpleConfig(PrintError):
         if not self.is_modifiable(key):
             self.print_stderr("Warning: not changing config key '%s' set on the command line" % key)
             return
+        try:
+            json.dumps(key)
+            json.dumps(value)
+        except:
+            self.print_error(f"json error: cannot save {repr(key)} ({repr(value)})")
+            return
         self._set_key_in_user_config(key, value, save)
 
     def _set_key_in_user_config(self, key, value, save=True):

electrum/storage.py

@@ -90,7 +90,7 @@ class JsonDB(PrintError):
             json.dumps(key, cls=util.MyEncoder)
             json.dumps(value, cls=util.MyEncoder)
         except:
-            self.print_error("json error: cannot save", key)
+            self.print_error(f"json error: cannot save {repr(key)} ({repr(value)})")
             return
         with self.db_lock:
             if value is not None: