Add validation that staging directory is writable

Problem/Motivation

found in #3277830: [DrupalCon] Automatic Updates beta test result by @AjitS

The temporary directory write issue in the container. It seems like the automatic_updates module tried to create a directory in the /tmp directory in the container and it was not able to do that. As a workaround, I changed the temporary directory for the Drupal site in settings.php using
$settings['file_temp_path'] = 'sites/default/files/tmp';

Proposed resolution

We should at least see what a problem like this would look like to the user. If there error is very hard to understand we might want create a validator

I suspect we probably will need to add some kind of check in Package Manager to ensure that the temporary directory (or "staging root" in Package Manager's jargon) is writable, to prevent folks from running into this error without recourse.

Remaining tasks

User interface changes

API changes

Data model changes