export: Convert a bunch of error cases to AssertionError.
export: Convert a bunch of error cases to AssertionError.

This reflects the fact that these are just defensive programming (we
don't expect them to ever happen) and also nicely makes these lines
not show up in our missing test coverage reports.

shubhamdhama/zulip,andersk/zulip,shubhamdhama/zulip,tommyip/zulip,rht/zulip,showell/zulip,zulip/zulip,andersk/zulip,dhcrzf/zulip,rht/zulip,zulip/zulip,dhcrzf/zulip,showell/zulip,brainwane/zulip,zulip/zulip,jackrzhang/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,shubhamdhama/zulip,dhcrzf/zulip,jackrzhang/zulip,shubhamdhama/zulip,dhcrzf/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,timabbott/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,kou/zulip,timabbott/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,rishig/zulip,kou/zulip,brainwane/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,hackerkid/zulip,shubhamdhama/zulip,dhcrzf/zulip,jackrzhang/zulip,dhcrzf/zulip,brainwane/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,rishig/zulip,eeshangarg/zulip,hackerkid/zulip,brainwane/zulip,hackerkid/zulip,tommyip/zulip,tommyip/zulip,tommyip/zulip,timabbott/zulip,jackrzhang/zulip,eeshangarg/zulip,rishig/zulip,jackrzhang/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,jackrzhang/zulip,shubhamdhama/zulip,andersk/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,hackerkid/zulip,tommyip/zulip,punchagan/zulip,hackerkid/zulip,rishig/zulip,zulip/zulip,hackerkid/zulip,jackrzhang/zulip,zulip/zulip,brainwane/zulip,dhcrzf/zulip,kou/zulip,rht/zulip,timabbott/zulip,kou/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,rishig/zulip,tommyip/zulip,rishig/zulip,shubhamdhama/zulip,tommyip/zulip,eeshangarg/zulip,rishig/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip