improve update error page: remove box, make link visible, insert missing space, remove line break

Resubmit of https://github.com/owncloud/core/pull/12753 – please review again @owncloud/designers @enoch85 @wakeup @LukasReschke 

Before:
![update before](https://cloud.githubusercontent.com/assets/925062/5376004/f664f67c-8070-11e4-828d-99b6e9dbee25.png)

After:
![error after](https://cloud.githubusercontent.com/assets/925062/5376005/f6651cc4-8070-11e4-9301-b1b07947f7b6.png)