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 only other place with a similar error is with the dependency on DOJO, having the following message:

message ( FATAL_ERROR \"Can't find Dojo build tools -- did you initialise submodules? (git submodule update --init --recursive)\" )

Perhaps we should start with a strict conformity before these become more common?