[store] Error handling in the `with_store` function

[illright]

The with_store function expects the last argument, the callback, to return a Result<T, Error>, where Error is a built-in error type. It's rather restricting because it doesn't allow for meaningful error handling on the user side.

For example (doesn't compile):

let value = tauri_plugin_store::with_store(
    self.handle,
    self.stores,
    SETTINGS_PATH.into(),
    |store| {
        store
            .get(key)
            .and_then(|value| value.as_u64())
            .or_else(|| self.defaults.get(key).and_then(|value| value.as_u64()))
    },
);

This callback returns Option<u64>, but the with_store function expects it to return Result<u64, Error>. I want to convert this Option into an error using .ok_or(), but I need a meaningful way to communicate that a value with a given key was not found in the store. None of the options of the Error enum satisfy that purpose:

pub enum Error {
    #[error("Failed to serialize store. {0}")]
    Serialize(Box<dyn std::error::Error + Send + Sync>),
    #[error("Failed to deserialize store. {0}")]
    Deserialize(Box<dyn std::error::Error + Send + Sync>),
    /// JSON error.
    #[error(transparent)]
    Json(#[from] serde_json::Error),
    /// IO error.
    #[error(transparent)]
    Io(#[from] std::io::Error),
    /// Store not found
    #[error("Store \"{0}\" not found")]
    NotFound(PathBuf),
    /// Some Tauri API failed
    #[error(transparent)]
    Tauri(#[from] tauri::Error),
}