Be more informative on dates found for hc thickness

For hc thickess, and diffs in particular it may be that one date is found but not the other. This is difficult to trace / debug som more information and possible exceptions would be beneficial.

Locate the hc thickness date-handling path, especially the diff case, and reproduce a situation where only one date is found. Define the missing-date diagnostics and exception behavior, then verify that the resulting messages clearly identify which date is absent and where processing stopped.

Written by the indexing model from the issue text.