Don't change status when cancelled downloads fail

It takes a bit of time to actually pause (i.e. cancel) a download. If
we change the update status once the download is actually cancelled,
we might override any status change that happened between the user
request and its completion. For example, the following can happen when
the user deletes an ongoing downloads:

 - pauseDownload(): DOWNLOADING -> PAUSED
 - deleteUpdate(): PAUSED -> DELETED
 - onFailure(): DELETED -> PAUSED

Don't change the status of cancelled downloads from the onFailure()
callback not to have an incorrect final status.

Change-Id: Iee90482d82df984672c59d7c508ad834fa08061f