manage.py validate not exiting with error if problems are found.

A recent merge request introduced borked jobs (with duplicated estimated_duration). These were missed by the containerized tests.

I ran ./manage.py validate in the affected provider manually, and the test failed as it should have (at least, found and reported the error), but manage.py validate eventually exits with success. ANd it even says "all jobs seem to be valid".

./manage.py validate && echo $?
WARNING plainbox.secure.plugins: Unable to prepare plugin /src/checkbox/git/providers/plainbox-provider-checkbox/jobs/stress.txt.in: Cannot load job definitions from '/src/checkbox/git/providers/plainbox-provider-checkbox/jobs/stress.txt.in': Job has a duplicate key 'estimated_duration' with old value '7200.0\n' and new value '7200.0\n' (line 4)
Cannot load job definitions from '/src/checkbox/git/providers/plainbox-provider-checkbox/jobs/stress.txt.in': Job has a duplicate key 'estimated_duration' with old value '7200.0\n' and new value '7200.0\n' (line 4)
NOTE: subsequent units from problematic files are ignored
All jobs seem to be valid
0

I'm not sure what is needed for this to properly halt a merge, but it'd be a good idea to think about enabling it soon :)