[UX] Misleading text about insufficient permissions when text format is missing/disabled

[klonos]

I am filing this as a follow-up to #1575, but this problem is pre-existing.

Steps To Reproduce

1. Go to /admin/config/content/formats and add a new text format.
2. Create a piece of content using that format.
3. Head back to /admin/config/content/formats and disable/remove the text format that you have added in step 1.
4. Edit the piece of content that you have created in step 2...

Actual behavior
The field that was using the text format is now disabled 👍 ...but the text warns about insufficient permissions:

Expected behavior
The text should explain that the reason why the widget is locked/disabled is because the text format that was used when creating it, is no longer available (has either been disabled, or completely removed).

[klonos]

...in https://github.com/backdrop/backdrop/blob/1.x/core/modules/filter/filter.module#L1309 we are checking elseif (!$user_has_access || !$format_exists) ...for both of these conditions, we are calling filter_form_access_denied(), which has a single message for both situations.

We should instead use different messages, and in the case where the format is unavailable, be specific about whether the format is disabled (which is recoverable), or completely removed (which is a dead end).