migrate: informative msg on re-attempted migration

If an operator runs `pd migrate` and then runs it again, the error
message should accurately describe the situation that the local state is
already migrated, and state that pd is refusing to proceed as
instructed.

Notably this failure occurs even if the `--force` flag is provided to
`pd migrate`, so I've updated the docstring on that flag accordingly.

crates/bin/pd/src/cli.rs

@@ -133,6 +133,7 @@ pub enum RootCommand {
         #[clap(long, display_order = 200)]
         comet_home: Option<PathBuf>,
         /// If set, force a migration to occur even if the chain is not halted.
+        /// Will not override a detected mismatch in state versions.
         #[clap(long, display_order = 1000)]
         force: bool,
         /// If set, edit local state to permit the node to start, despite

crates/core/app/src/app_version/component.rs

@@ -54,6 +54,14 @@ fn check_version(ctx: CheckContext, expected: u64, found: Option<u64>) -> anyhow
             let expected_name = version_to_software_version(expected);
             let found_name = version_to_software_version(found);
             let mut error = String::new();
+            if found == APP_VERSION {
+                write!(
+                    &mut error,
+                    "state already migrated to version {}",
+                    APP_VERSION
+                )?;
+                anyhow::bail!(error);
+            }
             error.push_str("app version mismatch:\n");
             write!(
                 &mut error,