rest: Simplify authentication error handling.
rest: Simplify authentication error handling.

This pure refactor removes a now unnecessarily nested else clause,
helping keep this key piece of code readable.

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