Assertion doesn't handle failed check

[dhalperi]

The questions underlying Batfish assertions can fail, in which case the Answer returned will not be a TableAnswer. In at least one case, the check is not handling this case well: instead of getting the error, we get:

fatal: [localhost]: FAILED! => {"changed": false, "msg": "1 of 1 assertions failed", "result": [{"details": "'Answer' object has no attribute 'frame'", "name": "Confirm routers can ping each other", "status": "Error", "type": "assert_all_flows_succeed"}], "summary": "1 of 1 assertions failed"}

(see: https://networktocode.slack.com/archives/CCE02JK7T/p1582840013048800)

Batfish should always check the answer type before calling .frame().