compiler/det_report.m:
In some cases described in the new test case, the error message
we print for a missing switch inside an arm of a larger switch
is misleading unless we print *which* arm of the larger switch
we are complaining about, so print that information.
tests/invalid/require_complete_nested_switch.{m,err_exp}:
A test case derived from the real-life error that motivated this diff.
tests/invalid/Mmakefile:
Enable the new test case.
