Terminal launch errors are not reported properly on Windows

[Tyriar]

Repro:

1. Add settings "terminal.integrated.shell.windows": "C:\\windows\\Sysnative\\"
2. Launch terminal

There should be an error message like on Linux/macOS. terminalProcess does not seem to notice that winpty exited.

[Tyriar]

Closing as duplicate of #31301 as there's more discussion there.