Check whether new synchronized directories exist and are empty.

src/bin/cli.rs

         // TODO: Login information.
         local_directory: PathBuf,
         remote_url: String,
+        #[structopt(short, long)]
+        force: bool,
     },
     RemoveDirectory {
         local_directory: PathBuf,

src/bin/client.rs

         errors: mpsc::Sender<SynchronizationError>,
     ) -> Result<SynchronizedDirectory, Error> {
         // TODO: First, connect to the server and test whether we can access the URL?
-        // At this point, the directory is still paused.
-        // TODO: Return an error if the directory is not empty (unless the user specified an
-        // override option).
+        // Probably not possible, at this point the directory is still paused.
         let file_tree = Arc::new(Mutex::new(FileTree::create(
             database, directory, remote_url,
         )?));
                                 stream.write_all(&(-1i32).to_be_bytes()).await.ok();
                                 stream.write_all(&output).await.ok();
                                 stream
-                                    .write_all(format!("Error: {:?}", e).as_bytes())
+                                    .write_all(format!("Error: {}\n", e).as_bytes())
                                     .await
                                     .ok();
                             }
         cli::Command::AddDirectory {
             local_directory,
             remote_url,
+            force,
         } => {
+            if !local_directory.exists() || !local_directory.is_dir() {
+                return Err(Error::InvalidParam(
+                    "the specified local path does not exist or is no directory".to_owned(),
+                ));
+            }
+            if local_directory.read_dir()?.next().is_some() && !force {
+                return Err(Error::InvalidParam(
+                    "the specified local directory is not empty (use --force to add the directory)"
+                        .to_owned(),
+                ));
+            }
             let local_directory = fs::canonicalize(local_directory)?;
             // We need to check that we are not synchronizing directories that are within
             // other synchronized directories.