darc add-default-channel should warn or error when the channel is not setup for publishing

roslyn and msbuild both had an issue during the recent 17.10.P3 snap, where contributors updated the darc config to publish their main branch builds to the "VS 17.11" channel. This was done because the component main and VS main branches were targeting VS 17.11.P1 after the snap.

However the "VS 17.11" channel was not set up for publishing and the subsequent official builds failed in the Publish Using Darc stage as a result, e.g. https://dev.azure.com/dnceng/7ea9116e-9fac-403d-b258-b31fcf1bb293/_build/results?buildId=2421415

This misconfiguration would have been prevented by a warning or error in the add-default-channel command which detects that the channel is not setup for publishing.

See also First Responders thread https://teams.microsoft.com/l/message/19:afba3d1545dd45d7b79f34c1821f6055@thread.skype/1712158633951?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=4d73664c-9f2f-450d-82a5-c2f02756606d&parentMessageId=1712158633951&teamName=.NET%20Core%20Eng%20Services%20Partners&channelName=First%20Responders&createdTime=1712158633951