Be more explicit with description homogenization issues

[psanker]

If you are homogenizing datasets with missing descriptions, panelcleaner throws an uninformative

 16.       └─panelcleaner:::tk_err("Wave {ui_value(w)} not found in mapping waves.")

(only identified with a traceback)

Full stack trace:

 13. └─panelcleaner::homogenize_panel(.)
 14.   └─panelcleaner:::homogenize_descriptions(panel, mapping, ctx = context)
 15.     └─panelcleaner:::homogenize_wave_descriptions(...)
 16.       └─panelcleaner:::tk_err("Wave {ui_value(w)} not found in mapping waves.")

The error message should make it clear that a description is missing.