Require a default branch for an action node

[jsccast]

See #44 and forthcoming code to address that issue.

[jsccast]

For now, if an action node follows no outgoing branch, the machine should transition its error node (even if such a thing doesn't exist). That's the standard behavior for the hopefully few possible error-like conditions. (Note that an error in an action execution can be handled in a slightly richer way.)