Squashed 'libertree-backend-rb/' changes from e3e9d27..d977aa0

d977aa0 update tests
f5feac3 with_tree: return response
775c0b1 reraise Libertree exceptions

git-subtree-dir: libertree-backend-rb
git-subtree-split: d977aa0d2a54c1f4ed514e73e498db54b061b474
Merge commit 'c59b2cae2f46e4f476a2318b552eb9c875b11870'
increase backend version