Improve error reporting in maybe_activate_stream_item.
Improve error reporting in maybe_activate_stream_item.

This change should lead to clearer tracebacks when our
assumption about the stream list's list items get
violated, and we also short circuit some code in the
caller that tries to scroll to the active stream.

verma-varsha/zulip,amanharitsh123/zulip,zulip/zulip,timabbott/zulip,rht/zulip,vabs22/zulip,vaidap/zulip,brockwhittaker/zulip,hackerkid/zulip,shubhamdhama/zulip,brainwane/zulip,verma-varsha/zulip,jrowan/zulip,shubhamdhama/zulip,brockwhittaker/zulip,tommyip/zulip,rishig/zulip,brainwane/zulip,Galexrt/zulip,synicalsyntax/zulip,rishig/zulip,vaidap/zulip,Galexrt/zulip,showell/zulip,showell/zulip,showell/zulip,jackrzhang/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,amanharitsh123/zulip,jackrzhang/zulip,eeshangarg/zulip,verma-varsha/zulip,shubhamdhama/zulip,amanharitsh123/zulip,vaidap/zulip,synicalsyntax/zulip,tommyip/zulip,eeshangarg/zulip,timabbott/zulip,mahim97/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,mahim97/zulip,hackerkid/zulip,punchagan/zulip,vabs22/zulip,mahim97/zulip,jrowan/zulip,shubhamdhama/zulip,rht/zulip,amanharitsh123/zulip,brockwhittaker/zulip,showell/zulip,brainwane/zulip,timabbott/zulip,showell/zulip,eeshangarg/zulip,rishig/zulip,tommyip/zulip,synicalsyntax/zulip,rishig/zulip,verma-varsha/zulip,rishig/zulip,jackrzhang/zulip,Galexrt/zulip,tommyip/zulip,jackrzhang/zulip,brockwhittaker/zulip,rht/zulip,timabbott/zulip,jackrzhang/zulip,brainwane/zulip,timabbott/zulip,shubhamdhama/zulip,dhcrzf/zulip,brockwhittaker/zulip,Galexrt/zulip,andersk/zulip,jackrzhang/zulip,kou/zulip,vabs22/zulip,vaidap/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,mahim97/zulip,punchagan/zulip,brainwane/zulip,verma-varsha/zulip,andersk/zulip,vaidap/zulip,kou/zulip,zulip/zulip,dhcrzf/zulip,hackerkid/zulip,andersk/zulip,amanharitsh123/zulip,showell/zulip,kou/zulip,dhcrzf/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,jackrzhang/zulip,rishig/zulip,verma-varsha/zulip,mahim97/zulip,hackerkid/zulip,jrowan/zulip,Galexrt/zulip,jrowan/zulip,dhcrzf/zulip,tommyip/zulip,amanharitsh123/zulip,punchagan/zulip,punchagan/zulip,synicalsyntax/zulip,showell/zulip,dhcrzf/zulip,andersk/zulip,dhcrzf/zulip,vabs22/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,synicalsyntax/zulip,brockwhittaker/zulip,kou/zulip,kou/zulip,andersk/zulip,zulip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,vaidap/zulip,Galexrt/zulip,tommyip/zulip,tommyip/zulip,dhcrzf/zulip,jrowan/zulip,zulip/zulip,zulip/zulip,timabbott/zulip,mahim97/zulip,vabs22/zulip,rht/zulip,vabs22/zulip,rht/zulip,jrowan/zulip,Galexrt/zulip,kou/zulip,timabbott/zulip,rht/zulip,kou/zulip,rishig/zulip,brainwane/zulip