Bluetooth: Mesh: Proxy: Downgrade advertising errors to warnings

When a small amount of supported connections is configured (especially
the default number of 1), connectable advertising may fail. This is
perfectly fine since as soon as a disconnection event happens the
advertising will be successfully restarted. To avoid causing
unnecessary user worries, downgrade the resulting errors to warnings.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>