Fix alive/ready status query functions

Fix omnisharp-check-alive-status, omnisharp-check-ready-status so that
they display correct message when server is not up.

(omnisharp--check-alive-status-worker)
(omnisharp--check-ready-status-worker): Only return t if the server
response is actually t, if it responds "Error communicating to the
OmniSharpServer instance" then formerly it would return t in that case
as well.