fdo#78949 Handle (SAXParse-)Exception that can't pass getAllRelationships

...like it was already done in the similar else branch of
ReadRelInfoIfNecessary.  The call site at GetAllRelationshipsIfAny will
translate an m_nRelInfoStatus that remained RELINFO_NOT_INIT into an
IOException.

Change-Id: I79c601acd23b3b0020c7b4de4c20b0282524bce1
(cherry picked from commit 423921b085de43f53e42f957889dd96378d2c3c4)
Reviewed-on: https://gerrit.libreoffice.org/9411
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>