[3.8] bpo-36871: Handle spec errors in assert_has_calls (GH-16005) (GH-16364)

[3.8] bpo-36871: Handle spec errors in assert_has_calls (GH-16005) (GH-16364)

The fix in PR 13261 handled the underlying issue about the spec for specific methods not being applied correctly, but it didn't fix the issue that was causing the misleading error message.

The code currently grabs a list of responses from _call_matcher (which may include exceptions). But it doesn't reach inside the list when checking if the result is an exception. This results in a misleading error message when one of the provided calls does not match the spec.

https://bugs.python.org/issue36871

Automerge-Triggered-By: @gpshead
(cherry picked from commit b5a7a4f0c20717a4c92c371583b5521b83f40f32)

Co-authored-by: Samuel Freilich <sfreilich@google.com>
https://bugs.python.org/issue36871

Automerge-Triggered-By: @gpshead