If btle connect attempt fails return QLEController back to Unconnected

What is really needed is a new ConnectionError for this case. This new
error will shortly be introduced to the btle/5.5 branch.

Change-Id: I751fa0eeb5cf4d0b78863842c94770f4e8e74015
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>