build: Check subprocess return codes
build: Check subprocess return codes

Pointed out (indirectly) by pylint's unused variable warning.

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@citrix.com>

simonjbeaumont/planex,djs55/planex,euanh/planex-cleanhistory,djs55/planex,euanh/planex-cleanhistory,jonludlam/planex,simonjbeaumont/planex,euanh/planex-cleanhistory,djs55/planex,jonludlam/planex,jonludlam/planex,simonjbeaumont/planex