Protect against crash in the `relation_possible` predicate with ambiguous relations. It was still possible to get a KeyError when using relations that are ambiguous at both ends, like (Executable, version_of, Program) & (Version, version_of, Project). Closes #3010148.

sync_schema_props_perms('cw_source', syncprops=False)
if schema['BigInt'].eid is None:
    add_entity_type('BigInt')