Output a more helpful error if operator is unavailable due to build features

[robertknight]

Some ONNX operators are only available if certain rten crate features are enabled. If rten is compiled with these features turned off and a user tries to run a model which requires them, they will get a generic error about the operator being unsupported, rather than telling them that they need to change the build settings. This might confuse them into thinking that RTen doesn't support the operator at all.

An example of affected operators are the Random* ops, which require enabling the random feature.

[robertknight]

A related issue is the case where a model was loaded with a subset of the normal operators available via Model::load_with_ops, and a user tries to load a model which requires an operator that was not included in the custom op registry.