cell: Prevent interpreter stack overflow from killing the domain

This no longer causes a dCache crash:

cd System
define env x
exec env x
.
exec env x

Now it simply outputs an error message stating that a stack overflow
happened. Before it caused the domain to restart.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7352/