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>
The weird indentation of these three lines is deliberate - cmake message is a little odd.