factor server_name change check into a reusable fn

src/error.rs

@@ -65,6 +65,16 @@ pub(crate) enum ServeCommand {
     #[error("failed to initialize services")]
     InitializeServices(#[source] crate::utils::error::Error),
 
+    #[error("`server_name` change check failed")]
+    ServerNameChanged(#[from] ServerNameChanged),
+}
+
+/// Error generated if `server_name` has changed or if checking this failed
+// Missing docs are allowed here since that kind of information should be
+// encoded in the error messages themselves anyway.
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub(crate) enum ServerNameChanged {
     #[error("failed to check if there are any users")]
     NonZeroUsers(#[source] crate::utils::error::Error),