If a bone is out of the mesh, Spine could have a hard time computing its weights and result in an error, try selecting just the main, necessary bones and click auto for those and then manually weight the problematic ones.
I once bound 40+ bones to a single mesh, Spine is strong, don't worry! (but try to have no more than 2-3 bones per vertex, and no weights below 2% as they're hardly noticeable and only make the animations heavier)