ConfdProcessor: move verification to __init__

During a query execution we currently verify whether we have that query
supported in our dispatch table. Since it's an error not to have it
anyway, we move this verification to the class creation, erroring out
early if such a problem exists.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>