Write traces on ::errorInfo no longer work under 8.5 I
due it seems to the refactoring of Tcl_AddObjErrorInfo,
and specifically the removal of Tcl_ObjSetVar2() from
The setting of a write trace on ::errorInfo lets a
debugger get control of an error before the stack
unwinds to inspect and modify the state/variables
interactively, ala gdb.