Bluetooth: Refactor check_pending_conn error handling

If anything went wrong just report connection failed. This makes
code easier to read and less error prone. Also makes sure that
scanning is reenabled with correct parameters on failure.

Change-Id: I7387571f6dbf308511694a635aa47d5371a81616
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>