use a distinct error for out-of-sync replica

cli/src/invocation/cmd/sync.rs

@@ -1,5 +1,5 @@
 use crate::settings::Settings;
-use taskchampion::{server::Server, Replica};
+use taskchampion::{server::Server, Error as TCError, Replica};
 use termcolor::WriteColor;
 
 pub(crate) fn execute<W: WriteColor>(
@@ -8,9 +8,32 @@ pub(crate) fn execute<W: WriteColor>(
     settings: &Settings,
     server: &mut Box<dyn Server>,
 ) -> Result<(), crate::Error> {
-    replica.sync(server, settings.avoid_snapshots)?;
-    writeln!(w, "sync complete.")?;
-    Ok(())
+    match replica.sync(server, settings.avoid_snapshots) {
+        Ok(()) => {
+            writeln!(w, "sync complete.")?;
+            Ok(())
+        }
+        Err(e) => match e.downcast() {
+            Ok(TCError::OutOfSync) => {
+                writeln!(w, "This replica cannot be synchronized with the server.")?;
+                writeln!(
+                    w,
+                    "It may be too old, or some other failure may have occurred."
+                )?;
+                writeln!(
+                    w,
+                    "To start fresh, remove the local task database and run `ta sync` again."
+                )?;
+                writeln!(
+                    w,
+                    "Note that doing so will lose any un-synchronized local changes."
+                )?;
+                Ok(())
+            }
+            Ok(e) => Err(e.into()),
+            Err(e) => Err(e.into()),
+        },
+    }
 }
 
 #[cfg(test)]