swift-ring-builder has three exit statuses: 0 (OK), 1 (WARNING),
2 (ERROR). Uncaught exceptions in python result in an exit code of 1,
so for example problems writing a builder file to disk will result in
an exit of 1 (warning) rather than 2 (error).
This addresses that by overriding sys.excepthook to produce the usual
backtrace and then exit 2 (error); excepthook is called when an
exception is unhandled, unless that is SystemExit.
Closes-Bug: 1960657
Change-Id: I7cfeff4f436ade319cf21d0d29853931aef6d20f
89ee843080