Fix missing coverage for aws_cryptosdk_priv_fail_session

Nothing currently ends up calling this in the tests. Probably we can consolidate some of the session_process codepaths to call it instead of open-coding the same sort of logic.

Locate aws_cryptosdk_priv_fail_session and the session_process codepaths, then inspect the existing tests that exercise those paths. Consolidate the relevant path so the function is called rather than duplicating its logic, and verify that tests execute the function and improve coverage.

Written by the indexing model from the issue text.