Fix lowenergyscanner when dealing with device scan errors

Previously the error was indicated but other UI elements were still
giving the impression that the scan was ongoing.

Change-Id: Iafab8f43bff3303eb6ad5e3385db2f81288d2e66
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>