migration: show error message when postcopy fails

migration: show error message when postcopy fails

The 'info migrate' command only shows the error message when the
migration state is 'failed'. When postcopy is used, however,
the 'postcopy-paused' state is used instead of 'failed', so we
must show the error message there too.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/qemu-devel/20250721133913.2914669-1-berrange@redhat.com
[line break to satisfy checkpatch]
Signed-off-by: Fabiano Rosas <farosas@suse.de>