dazl.ledgerutil.ACS methods should throw exceptions (or return warnings) when asked for a non-existent template

[da-tanabe]

The following code snippet silently returns an empty dict, and it probably could do something more helpful:

queries = {"Some:Tmpl": None}
async with ACS(..., queries) as acs:
    snapshot = await acs.read()

    # returns an empty dict, but doesn't warn or raise an error
    snapshot.matcing_contracts("Some:Tmpl2")

These calls will always fail, and this can be detected; some way of informing the user that they made a mistake would be useful here.