Bluetooth: Add error handling for managment command handlers

The command handlers for bluetooth management messaging should be able
to report errors (such as memory allocation failures) to the higher
levels in the call stack.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>