Add some kind of input validation to table dumps

[bdpedigo]

MaterializationEngine/materializationengine/blueprints/materialize/api.py

Line 419 in 70f85f6

def post(self, datastack_name: str, version: int, table_name: str):

would be nice if this checked at least that the table exists and that kind of thing - currently there is no error message (and in fact, the 0 bytes header file is created) if you ask for a dump of a table that doesn't exist