Merge lp://staging/~mvo/software-center/no-piston-data-means-error into lp://staging/software-center

Hi Michael, thanks very much for this. As I mentioned in lp:~gary-lasker/software-center/display-cached-recommendations, relying on this error condition has the unwanted side-effect in Software Center of inexplicably hiding the lobby view's recommendations panel completely for the error case.

Therefore, I made a different fix in that branch that correctly detects the condition and displays the standard "Recommendations will appear when next online" message in the panel.

This branch, then, in not strictly needed anymore (at least for the offline recommendations case), but it seems to me that it's a good general approach to return an error if piston does not have cached data available, so I would approve this for merging if you still think it's a good idea as well.

Many thanks!

Here is the corresponding merge proposal for the branch that I referred to in my previous comment:

  https://code.launchpad.net/~gary-lasker/software-center/display-cached-recommendations/+merge/119614

Michael points out that a similar endless spinner behavior occurs when an appdetails page is viewed under the conditions described above, and this branch fixes that (in that case, it is correct behavior to simply hide the panel). I verified that this branch fixes this issue. Therefore, we should definitely merge this.

Thanks again mvo!