[web] Allow propagating 'klass' parameter on tree views recursively Closes #4881302

[utils] Fix define_var in ajax context (closes #4881301) Use namespaced <cubicweb:script> instead of just <script>

[web/data] Never hide JS errors in our Deferred implementation Modern browsers have good debugging tools, let exceptions go all the way up. This only affects exceptions raised in the js code, if there's an issue on the server/python side, they'll keep going through our error callbacks. Closes #4881300