[registry] backout 120c7ea6e1fa: errors should not pass silently
Catching ImportError silently while loading files has too many side effects.
(e.g. if system hooks / entities can't be registered).
# type attribute might already be there if migrating from
# version < 3.5 to version >= 3.5.3, BaseTransition being added
# in bootstrap_migration
if versions_map['cubicweb'][0] >= (3, 5, 0):
add_attribute('BaseTransition', 'type')
sync_schema_props_perms('state_of')
sync_schema_props_perms('transition_of')