langref- fix packed struct error code

[nektro]

The current docs say the error for this section is docgen_tmp/test.zig:8:11: error: unused local constant. However, they also mention that it should be failing due to #1994 instead. This change uses the variable so that the proper error can propagate.

[Vexu]

Could you add the expected error message using the test_err|expected type... syntax to both of your prs so that they don't regress again.

There also seem to be a few other places that are missing the error if you want to fix those too.