slack import: Improve error handling for invalid messages.
slack import: Improve error handling for invalid messages.

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