[2.x] Reset save button state when an error occurs in admin page

[YUCLing]

Fixes #3962

Changes proposed in this pull request: This PR contains changes to AdminPage component, which makes sure the state of the save button is reset if an error occurs.

Reviewers should focus on: Is the implementation correct and matches the standard way of Flarum?

Screenshot None

QA Trigger a server-side error while saving settings in admin page, check if the state of the save button is reset.

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

• [x] Frontend changes: tested on a local Flarum installation.
• [ ] Core developer confirmed locally this works as intended.
• [ ] Tests have been added, or are not appropriate here.