shell: Polish create volume group dialog

Add warning when there are no available disks
Validate the length and content of input
Add visual style when error occurs
Check the precondition of creating a volume group

Fixes #871
The above four functions shouldn't be methods of this class. They don't operate on class members, nor are they specific to this class. These functions should be simple functions inside the private cockpit-storage.js scope:

 * highlight_error
 * hide_error
 * highlight_error_message
 * hide_error_message