Missing windowing error message is not undestandable to Euphoria user

RBK a JOIN cannot wotk witout windowing when inputs are unbounded. Translators do not reports it, but underalying Beam transforms do: `GroupByKey cannot be applied to non-bounded PCollection in the GlobalWindow without a trigger.` Their description of the problem is hart do understind for Euphoria API user.  We should issue errror messahe of our own. Napr. `ReduceByKey cannot be used without windowing when input is unbounded.`