HTTP code should be set to 400 in case of improper payload sent to the stack_license endpoint

[tisnik]

Currently, the stack_license endpoint does not contain any check if the payload sent to this endpoint is correct. It means that the service fail later with HTTP code 500, and not with HTTP code 400.

Possible fix:

1. check payload right after it is received
2. respond accordingly

Report made by BAF can be seen here:
https://fabric8-analytics.github.io/fuzz-tests/recommender_stack_license_issue_168.htm