Sets command cmd as the procedure to call when
an error occurs in a callback.  When an error
occurs, SCO Visual Tcl will go up the object tree until it finds an
error callback to handle the error. 
Additional callback keys:
callback- 
Callback that caused the error.
 
errorCode- 
Return code from Tcl eval.
 
result- 
Tcl Result string, the error string.