data_import: Protect better against bad Slack tokens.
data_import: Protect better against bad Slack tokens.

An invalid token would be treated the same as a token with no scopes;
differentiate these better.

andersk/zulip,punchagan/zulip,rht/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,kou/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,zulip/zulip