test_classes.py: Replace markdown_logger mock with assertLogs.
test_classes.py: Replace markdown_logger mock with assertLogs.

Set level to 'ERROR' since exceptions create logs with that level.

zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,rht/zulip