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.
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc