There is no* safe condition used by Ariane 5.
It is not common to use Launch Escape Systems for UNmanned flights. Actually the hardware of a spacecraft is not as costly as you may think, the development itself is driving the costs. So usually there are Flight Spare Units (Wikipedia Flight Spare) of the spacecraft, sometimes even (nearly) assembled. If a launch failed, you put the spare parts together and try a second time. You just have to pay the man hours to put everything together. No additional development, no new coding, no manufacturing of new parts.
Of course Ariane-Space and the JWST-Guys have contracts if there are penalties payments due or have insurances to cover some damage. But in the end, that's cheaper than trying to save the (unmanned) payload.
*) I mean, there are several safe mechanisms, but not to save the payload, if there is a total loss of the rocket.
At least for some parts of JWST, information about Spare Models are available:
A CRYOGENIC ROTATING ACTUATOR
Some Prisms for "MIRI"