[fix] can't load webui if selected wrong extra option in ui

[light-and-ray]

Description

There is a bug: you can add an option from extension, e.g. controlnet_show_batch_images_in_ui, and after restart webui won't able to start, due to KeyError. It happens because this option was declared in on_ui_settings callback. I've found the solution, if the author of extension wants to be able to add their option in settings in ui, they need to use option template instead of callback, like w-e-w does in their nudenet_nsfw_censor_scripts/settings.py (fix in my extension: light-and-ray/sd-webui-lama-cleaner-masked-content@e8a9170) Maybe it's needed to add recommendation to not use on_ui_settings callback

My PR fixes 2 things:

• prevent webui crashing if can't find settings (e.g. extension was disabled)
• prevent user to add settings which are not available on create_ui moment

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

This can also be remedied by calling script_callbacks.ui_settings_callback() earlier; this way, controlnet's settings can also be added to the UI. A possible problem is that calling those callback earlier can break some scripts; I made the PR with changes: #15121