[server] Replace invalid sidebar source message with unsupported message

Summary: It could be possible for a sidebar source to reference not existing message. In that case, we could log a warning and display an UNSUPPORTED message instead.

Test Plan: Set initial message id to some not existing value and verify that the message is displayed.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D590