Found this in 1.3:
<StringTable>
<Label>TECH_VALIDATION_ERROR_LABEL</Label>
<String>ERROR: Tech Tree Audit: Tech Tree Fracture detected. This tech has no valid prerequisite, and is unreachable.[BR]Tech Name: %s[BR]Tree: %s</String>
</StringTable>
Looks like there might be something to actually tell us exactly where a custom tech tree has screwed up. That's a nice touch 