use a distinct error for out-of-sync replica

taskchampion/src/errors.rs

@@ -4,6 +4,12 @@ use thiserror::Error;
 #[non_exhaustive]
 /// Errors returned from taskchampion operations
 pub enum Error {
+    /// A task-database-related error
     #[error("Task Database Error: {0}")]
     Database(String),
+    /// An error specifically indicating that the local replica cannot
+    /// be synchronized with the sever, due to being out of date or some
+    /// other irrecoverable error.
+    #[error("Local replica is out of sync with the server")]
+    OutOfSync,
 }