Add push failure check
This merge request adds the test case needed to verify that the push failed properly when trying to send an invalid asset.
As benefit these tests have shown that some checks where passing but not using the correct class but they have been moved earlier in the chain so that if get_dependencies is called, the validity check was already done prior to it. This allows to reduce the code as well as centralise the check where it makes the most sense.
Also improved are the other push tests. They are now properly running/failing for the correct reasons.
Fixes #65 (closed)