Restores keyboard accessibility for expandable settings area.

Tabindex="0" makes the element focussable with the keyboard and
role="link" tells assistive technologies that this clickable span
element is supposed to be presented as a hyperlink.
Add error handling to the file cache upgrade
Merge pull request #2383 from owncloud/error-handling-upgrade

Add error handling to the file cache upgrade
Merge pull request #2380 from henningoschwald/76bb5221c7a1b71f0d617c55e844c51e889a2c70

Fixes keyboard accessibility of expandable settings