[topgen] Fail more understandably if we start in an unexpected place

[rswarbrick]

This should fix issue #24595. The problem that caused the issue is that we compute paths to the various IP blocks that should be included by following relative paths from the -t argument. This doesn't work very well if the top_earlgrey.hjson file moves around!

A sensible design would probably point more explicitly at where to find IPs, but this should hopefully be a little easier to debug.