Misleading status messages when downloading definitions

1. Go to "Edit" -> "Preferences" -> "Text Editor" -> "Generic Highlighter".
2. Disconnect your machine from the network.
3. Click "Download Definitions".
   In the Preferences dialog you can read "Download finished". In the General Messages view, you can read:

   Highlighter updates: starting
   Highlighter updates: done

   This suggests that the download was successful which it wasn't. Only when you started Creator from the command line, there you can read something like: "kf.syntaxhighlighting: QNetworkReply::HostNotFoundError".

Creator should check whether the download was successful and clearly indicate that to the user.

This was found with Squish test tst_generic_highlighter in Jenkins.