runner: add --keep-going flag, default false
runner: add --keep-going flag, default false

Allow control of whether `runner` should keep going even when it encounters
an error, analogous to the `set -e`/`set +e` flag for shell scripts.

Default to false so that an error immediately stops execution, for two reasons:

 - In interactive use (e.g. with `run-examples --client-variant=none`), any errors
   will scroll off-screen and then a stale version of the binaries (from a previous
   successful run) can be run without the user noticing.
 - In CI use, early termination leads to faster CI runs when there are failures.

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