vndk-def: Ignore missing /vendor/default.prop

This commit catches FileNotFoundError so that VNDK definition tool can
work on the early devices that do not have /vendor/default.prop.

Test: vndk_definition_tool.py deps-insight \
  --system ${BULLHEAD_OC_MR1}/system \
  --vendor ${BULLHEAD_OC_MR1}/vendor

Change-Id: Ia87e5a80894c5f0ab3997dddfbb0332ed1ae5aef