Better debug message on loading bad vehicle parts

[irwiss]

Summary

None

Purpose of change

Give slightly better message when loading a save with invalid vpart ids; Instead of vomiting json dump give the invalid part id in the message, then vomit the json

Describe the solution

Describe alternatives you've considered

Testing

Load old save that has no vpart migrations/modded game etc with invalid vpart id, the invalid id should be visible in the message itself

Additional context