Hackathon issue: crash if pilot proxy has no token in diracx config

[aldbr]

Description:

• A given user's proxy, from a given VO is used to submit pilot-jobs in Dirac
• The VO admin adds the VO section in DiracX config but forget to include the given user's subject.

Result: The SiteDirector tries to get a proxy from the given user, and - under the hoods - calls DiracX to exchange the proxy against a token, which cannot exist because the user's subject is not part of the config. At this point, DiracX just crashes again and again, affecting all the other VOs.