Make action validation

[meldckn]

Fill in the stubbed function, action(), in js/ensemble/Validate.js, line 144, that checks the structure of loaded action JSON and throws helpful error messages if it is malformed. See also parseAction() in js/ensemble/ActionLibrary.js, line 94, where validate.action() is called.

Look at semantics of action scoring in Ensemble core (also in ActionLibrary.js) before doing this to make sure it's consistent.