broadband-modem: run implicit disabling if enabling fails

The disabling sequence is updated so that the steps to disable the interfaces never fail. This is done so that the modem is not left in an "inconsistent" enabled state, if e.g. the modem is enabled and one of the disabling steps for the interfaces ends up failing. In this case, it is preferred to say that the modem is disabled, than having it wrongly enabled.

The enabling sequence is updated so that if any of the steps to enable the interfaces fail, we end up running an implicit disabling operation to disable all the interfaces. This is to attempt to cleanup whatever we had enabled during the enabling operation, including e.g. the open ports context.