Accept loop has improper error handling, causing wrong exit code.

When the proxy receives a SIGTERM, it will close this server socket. This can result in the server.Accept() returning a non-nil error with the message
"proxy server error: accept tcp" or "use of closed network connection". This error will be put into the error channel, causing the process end with exit code 2, instead of the appropriate 143 (TERMINATED) or 0 (SUCCESS).

See https://github.com/GoogleCloudPlatform/cloud-sql-proxy/blob/main/cmd/root.go#L1143