[1.x] fix: reset admin page save button in catch handler

[YUCLing]

Fixes #3962

Changes proposed in this pull request:
Add a catch handler to saveSettings() to reset admin page's save button state.

Reviewers should focus on:
Should a newly added function that only sets loading to false be created.

Screenshot
No much changes will be shown to users.

QA
Create a Saving event listener and throw an exception to create a saving error.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[YUCLing]

Should I also create a PR to fix this in 2.0?