Debugger stays active when program is missing

[Simmo Saan]

The only awkward behavior I noticed was that the debugger doesn't terminate if the a.out file (from the default launch configuration) doesn't exist. The integrated terminal will show a missing file error, but the VS Code debugging UI still stays active. I'm not sure there's a way to present an VS Code error in this case, but it might be worth looking into in the future.

Originally posted by @mnxn in ocamllabs/vscode-ocaml-platform#1148 (review)