Wibbles to error messages, following the fix for Trac #7851

In effect, the error context for naked variables now takes up
a "slot" in the context stack; but it is often empty.  So the
context stack becomes one shorter in those cases.  I don't think
this matters; indeed, it's aguably an improvement.  Anyway that's
why so many tests are affected.
Test Trac #7778
Test Trac #7851
Test Trac #7856