Correct errors in bin/install_plugins output

scripts/shared_functions.sh

@@ -57,7 +57,8 @@ reload_tmux_environment() {
 plugin_already_installed() {
 	local plugin="$1"
 	local plugin_path="$(shared_plugin_path "$plugin")"
-	cd "$plugin_path" &&
+	[ -d "$plugin_path" ] &&
+		cd "$plugin_path" &&
 		git remote >/dev/null 2>&1
 }