eframe: panic if user does not have Permission to write to local app storage

[jleibs]

Describe the bug
If the user does not have access to create a file in their app directory, eframe panics on an unwrap.

To Reproduce

Remove your existing app storage, and then modify your app directory to prevent user access.

rm -rf /home/jleibs/.local/share/rerun/app.ron
sudo chown root.root /home/jleibs/.local/share/rerun/

Expected behavior
A warning is printed but the app doesn't panic.

Screenshots

Desktop (please complete the following information):

• OS: Ubuntu 22.10

Additional context

thread 'ThreadId(24)' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }', eframe-0.22.0/src/native/file_storage.rs:84

   6: core::panicking::panic_fmt
             at core/src/panicking.rs:64:14
   7: core::result::unwrap_failed
             at core/src/result.rs:1750:5
   8: <eframe::native::file_storage::FileStorage as eframe::epi::Storage>::flush::{{closure}}