import: Log a warning when avatars can't be thumbnailed.
import: Log a warning when avatars can't be thumbnailed.

This fixes a potential crash in the import tool if a single user has a
broken avatar image.

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