🐛 fix(error): add specific error for missing label in mailbox

- add `LableNotFoundInMailbox` error to handle cases where a label is not found in the mailbox

src/error.rs

@@ -18,6 +18,9 @@ pub enum Error {
     /// No rule for label
     #[error("No rule for label {0}")]
     NoRuleFoundForLabel(String),
+    /// No label found in the mailbox
+    #[error("Label {0} not found in the mailbox")]
+    LableNotFoundInMailbox(String),
     /// Rule not found for ID
     #[error("No rule for id {0}")]
     RuleNotFound(usize),