Add an explanation to error on partial overlay

I spent 4 hours searching for the bug and possible workarounds before I
figured this one out:

Fatal Error: target <MODULE>_generate_messages_cpp already defined in module
<SEEMINGLY_UNRELATED_MODULE>

We shouldn't really do anything here but error out, because it's a bad idea to
partially overlay the module in the new workspace - Everything in alphabet
before <MODULE> in the workspace which transitively relies on <MODULE> uses
the old one instead of the overlay..

But the old error message is pretty unhelpful and confusing.