Yep, it's over. I got the same error. In the game directory it's pretty much empty now but there's a text file message dated 1/9:

MechWarrior!

Thank you for your participation in the BattleTech Backer Beta! While it is a little sad that the time for playing this version of the game has come to an end, you can bet that means the full version is coming soon..."How soon?" you may ask. Soon - thank you for your patience as we wrap up development and stay tuned for more information.

We have learned a lot from your feedback and just the fact that you played has helped make the game better. We very much appreciate the playtime you put in and hope you enjoy the adjustments we have made when the full game arrives. We look forward to your playing at that time just as we know you are!

Best wishes,
Your Friends at Harebrained Schemes