Convert panics into error returns in load.rs

[KarelPeeters]

Currently load.rs can panic for many reasons:

• An invalid onnx file was given
• An unsupported operator was used
• An unsupported combination of operator/shape/attributes was used.

None of these should cause panics, these should all be errors with human-readable explanations. It doesn't have to be a nice enum type, just strings is already a good start. Panics should only be triggerable by implementation bugs.