Do not authenticate JWT if party-entity cannot be found

[DrillSergeant]

Current situation
When a JWT token is valid, a party object is expected to be found with the identifier of the token payload. When this object is not to be found, an exception occurs which leads to an 500 response code

Possible reasons for a party entity not to be found

• User (party) was deleted
• Account was moved to a differnt user (party)

Expected behaviour
In both cases the JWT should not be valid anymore and the request should result as "unauthorized" instead of 500.