scripts: ignore remote protocol checks if pdwtags crashes

On Debian 10, pdwtags reliably segfaults when parsing the libvirt remote
protocol files. This crash was previously ignored by 'make check'
because of the way we piped the pdwtags output to the perl
post-processing scripts. When this was converted to use python it
mistakenly started being a fatal error. We need to explicitly ignore
pdwtags output if it exited with non-zero return code.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>