front: add error messages for missing path steps in new itinerary modal

[Akctarus]

Closes #15950.

The translation keys are duplicated, but either that or modify the keyPrefix and editing all the translation calls in the file.

We don't handle train composition because it should be possible to add a train without selecting a rolling stock.

[RomainValls]

Thanks for the PR !

Overall it seems great, but here are two things :

• if I add an invalid pathStep, for example "toto" in the destination, I have an error "destination is not defined". -> Do we want to keep this message, as we do have a destination, but it just is invalid ? @maelysLeratRosso do you have an idea ?

• if I have a valid origin, and that I delete the input value of this pathstep, the error message is "At least two path steps are required to define an itinerary". It is correct, but maybe we should rater display a "missing origin" message.

[maelysLeratRosso]

Thanks for the PR !

Overall it seems great, but here are two things :

• if I add an invalid pathStep, for example "toto" in the destination, I have an error "destination is not defined". -> Do we want to keep this message, as we do have a destination, but it just is invalid ? @maelysLeratRosso do you have an idea ?

Maybe if the destination is invalid we should instead have the message we use for invalid path steps

• if I have a valid origin, and that I delete the input value of this pathstep, the error message is "At least two path steps are required to define an itinerary". It is correct, but maybe we should rater display a "missing origin" message.