Source

File: wp-admin/includes/class-wp-upgrader.php

public function generic_strings() {
		$this->strings['bad_request']       = __( 'Invalid data provided.' );
		$this->strings['fs_unavailable']    = __( 'Could not access filesystem.' );
		$this->strings['fs_error']          = __( 'Filesystem error.' );
		$this->strings['fs_no_root_dir']    = __( 'Unable to locate WordPress root directory.' );
		$this->strings['fs_no_content_dir'] = __( 'Unable to locate WordPress content directory (wp-content).' );
		$this->strings['fs_no_plugins_dir'] = __( 'Unable to locate WordPress plugin directory.' );
		$this->strings['fs_no_themes_dir']  = __( 'Unable to locate WordPress theme directory.' );
		/* translators: %s: Directory name. */
		$this->strings['fs_no_folder'] = __( 'Unable to locate needed folder (%s).' );

		$this->strings['download_failed']      = __( 'Download failed.' );
		$this->strings['installing_package']   = __( 'Installing the latest version…' );
		$this->strings['no_files']             = __( 'The package contains no files.' );
		$this->strings['folder_exists']        = __( 'Destination folder already exists.' );
		$this->strings['mkdir_failed']         = __( 'Could not create directory.' );
		$this->strings['incompatible_archive'] = __( 'The package could not be installed.' );
		$this->strings['files_not_writable']   = __( 'The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.' );

		$this->strings['maintenance_start'] = __( 'Enabling Maintenance mode…' );
		$this->strings['maintenance_end']   = __( 'Disabling Maintenance mode…' );
	}