[DebugInfo] Correctly report header parsing errors from DWARFContext::fixupIndex (#69505)

In ef762e5e7292, I shifted around where errors were reported when
failing to parse and/or validate DWARFUnitHeaders. When we are doing so
in DWARFContext::fixupIndex, the actual error message isn't prefixed
with `warning:` like it would be elsewhere (because of the way
`logAllUnhandledErrors` is implemented).