HPCC-12172 Better error message if required git update steps not done

When building the cassandra plugin, if a git submodule init has not
been done you get an unhelpful error message.

This should make it a bit less unhelpful.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6442 from richardkchapman/cassandra-cmake-error

HPCC-12172 Better error message if required git update steps not done

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>