Unclear location for missing unit errors

[Christian-W. Budde]

When a program failed to compile because of missing units, the missing units are reported to be located at the very last spot of the unit section. If possible it would be nice if the units could get located more precisely. In this case the user could spot the missing units more easily, especially if the IDE uses underlining of errors like in VSCode.

[Eric Grange]

Fixed issue #125, improved error location for units in "uses" clauses + updated tests

→ <<cset d37cf2ff5a14>>

[Christian-W. Budde]

Thanks, seem to work fine!