settings: Show notification when user/project settings fail to parse (#18122)

Closes #16876

We only ever showed parsing errors, but not if something failed to
deserialize.

Basically, if you had a stray `,` somewhere, we'd show a notification
for user errors, but only squiggly lines if you had a `[]` instead of a
`{}`.

The squiggly lines would only show up when there were schema errors.

In the case of `formatter` settings, for example, if someone put in a
`{}` instead of `[]`, we'd never show anything.

With this change we always show a notification if parsing user or
project settings fails.

(Right now, the error message might still be bad, but that's a separate
change)


Release Notes:

- Added a notification to warn users if their user settings or
project-local settings failed to deserialize.

Demo:


https://github.com/user-attachments/assets/e5c48165-f2f7-4b5c-9c6d-6ea74f678683

crates/settings/src/settings.rs

@@ -13,7 +13,9 @@ pub use editable_setting_control::*;
 pub use json_schema::*;
 pub use keymap_file::KeymapFile;
 pub use settings_file::*;
-pub use settings_store::{Settings, SettingsLocation, SettingsSources, SettingsStore};
+pub use settings_store::{
+    InvalidSettingsError, Settings, SettingsLocation, SettingsSources, SettingsStore,
+};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash, PartialOrd, Ord)]
 pub struct WorktreeId(usize);