lightbox: Add debugging code for unknown message IDs.
lightbox: Add debugging code for unknown message IDs.

This should help debug a traceback we've been seeing in production
where the fetched message was undefined.

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