Fix bug in last commit: exit in function call is OK.

This made the osh-runtime benchmarks fail.  Make the error stand out
more.

benchmarks/osh-runtime.sh

@@ -246,7 +246,7 @@ measure() {
  # An empty pattern matches every line.
  time egrep "$pattern" $tasks |
    xargs -n $NUM_COLUMNS -- $0 runtime-task $raw_dir ||
    die "Some tasks failed."
    die "*** Some tasks failed. ***"

  cp -v $provenance $raw_dir
}

core/cmd_exec.py

@@ -1381,8 +1381,10 @@ def RunFunc(self, func_node, argv):
    except _ControlFlow as e:
      if e.IsReturn():
        status = e.StatusCode()
      elif e.IsExit():
        raise
      else:
        # break/continue used in the wrong place
        # break/continue used in the wrong place.
        e_die('Unexpected %r (in function call)', e.token.val, token=e.token)
    finally:
      self.mem.PopCall()

spec/builtins.test.sh

@@ -89,7 +89,9 @@ echo status=$?
# OK dash stdout: status=127

### Exit builtin
exit 3
f() { exit 3; }
f
exit 4
# status: 3

### Exit builtin with invalid arg