Driver diagnostics are confusing when you have multiple home units

Currently the driver diagnostics don't give any indication about which unit they correspond to.

For example -Wmissing-home-modules can fire multiple times for each different home unit and gives no indication about which unit it's actually reporting about.

Perhaps a longer term fix is to generalise the providence information away from a SrcSpan so that these kind of whole project errors can be reported with an accurate provenance. For now we can just include the UnitId in the error message.