restrict with-exit-codes to "run", "bash" and "system"

[nojb]

This PR follow a remark by @aalekseyev that the current semantics of with-exit-codes are not quite OK: in (with-exit-codes 2 (progn (exit 2) do_thing), do_thing will run, which is not expected.

Some alternatives were discussed:

1. fix the behaviour: this means propagating the exit code of each run so that the enclosing action can decide whether to continue sequentially or simply return until the nearest with-exit-codes handler is found.

2. switch to a special action (with-accepted-exit-codes <codes> <prog> <args>)

3. add an optional argument to run to specify accepted exit codes.

I investigated 1), but the change seemed to complicate the code throughout simply to support this feature. It wasn't clear to me that the tradeoff was worth it. On the other hand, extending the syntax as in 3) is a bit tricky and needs more discussion.

So I implement 2) in this PR.

[aalekseyev]

A fourth plausible alternative is to keep the syntax as is, but make it an error to use with-accepted-exit-codes with anything except run, bash and system.

[nojb]

A fourth plausible alternative is to keep the syntax as is, but make it an error to use with-accepted-exit-codes with anything except run, bash and system.

Good idea, this is a bit more flexible but still avoids the issue in question. I will wait a bit to see if there are more opinions, but this seems like a good compromise.

[nojb]

I reworked the PR along the suggestion of @aalekseyev (and renamed with-exit-codes => with-accepted-exit-codes). Review welcome !

[aalekseyev]

Looks good to me.