Coder CLI needs a clearer error when it connects to an incompatible server #13478
Description
If you connect with v2.12 CLI to an earlier Coder Server, with any command that connects to the agent (coder ssh, coder speedtest, coder port-forward, etc.), you'll see
2024-06-05 11:31:59.362 [erro] failed to dial tailnet v2+ API error="failed to WebSocket dial: expected handshake response status code 101 but got 400"
2024-06-05 11:31:59.572 [debu] net.wgengine: ping(fd7a:115c:a1e0:405d:acae:5d4e:7d41:a808): no matching peer
2024-06-05 11:31:59.706 [debu] dialing Coder tailnet v2+ API
2024-06-05 11:31:59.760 [erro] failed to dial tailnet v2+ API error="failed to WebSocket dial: expected handshake response status code 101 but got 400"
2024-06-05 11:31:59.872 [debu] net.wgengine: ping(fd7a:115c:a1e0:405d:acae:5d4e:7d41:a808): no matching peer
2024-06-05 11:32:00.316 [debu] dialing Coder tailnet v2+ API
2024-06-05 11:32:00.368 [erro] failed to dial tailnet v2+ API error="failed to WebSocket dial: expected handshake response status code 101 but got 400"
2024-06-05 11:32:00.512 [debu] net.wgengine: ping(fd7a:115c:a1e0:405d:acae:5d4e:7d41:a808): no matching peer
2024-06-05 11:32:01.200 [debu] net.wgengine: ping(fd7a:115c:a1e0:405d:acae:5d4e:7d41:a808): no matching peer
2024-06-05 11:32:01.266 [debu] dialing Coder tailnet v2+ API
2024-06-05 11:32:01.318 [erro] failed to dial tailnet v2+ API error="failed to WebSocket dial: expected handshake response status code 101 but got 400"
I'm pretty sure the server is writing a reasonably helpful HTTP response with an error in the body, but here we only log the status code and it's not helpful.