Stash away more backtraces

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Rather than simply comment 'will exit with code 2', we exit with code 2

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xe: add a standard parameter '--trace' to print backtraces on stderr

Signed-off-by: David Scott <dave.scott@eu.citrix.com>