Cancel run on extension/plugin errors instead of running without mirrord

[t4lz]

If something goes wrong before starting the run with mirrord, we don't load into the process, but we do let the program start without mirrord.
I think we should instead cancel the run altogether.

If a user is trying to run with mirrord, running without mirrord is not going to help them at all.
Moreover, it might cause problematic situations, for example if the program has side effects that are meant to happen remotely - it might be bad for them to happen locally (e.g. overriding files).

I didn't check what happens on vscode, please edit this issue accordingly if you know.