HPCC-11898 Roxie should support superfiles with >1 file

Roxie gives an assert error on trying to read superfiles with >1 subfile. This
should be changed so that FETCH and KEYED JOIN throw a more meaningful error,
and other disk reads just work (since there is no technical reason they
can't)...

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>