boxes: Improve error message for invalid stream name.

Now that autocomplete for stream box has been added, we can show a hint
to use autocomplete when the user inputs an invalid stream name.

Author: kaustubh-nair

zulipterminal/ui_tools/boxes.py

@@ -319,7 +319,13 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
                     if header.focus_col == 0:
                         stream_name = header[0].edit_text
                         if not self.model.is_valid_stream(stream_name):
-                            self.view.set_footer_text("Invalid stream name", 3)
+                            invalid_stream_error = (
+                                'Invalid stream name.'
+                                ' Use {} or {} to autocomplete.'
+                                .format(keys_for_command('AUTOCOMPLETE').pop(),
+                                    keys_for_command('AUTOCOMPLETE_REVERSE').pop())
+                            )
+                            self.view.set_footer_text(invalid_stream_error, 3)
                             return key
                         user_ids = self.model.get_other_subscribers_in_stream(
                                                     stream_name=stream_name)