Scadnano import error message could be more informative.

[cgevans]

Currently, in most cases when scadnano import fails, ENSnano gives the error message Error when loading design: "Oh No" (https://github.com/thenlevy/ensnano/blob/7561224887df846cdb6c975a42b16ef1f55d10c0/src/main.rs#L1348). It looks like the loading process, however, does have more specific error information (eg, ScadnanoImportError). If it would be possible to display these details on failure, it would be helpful: I often have scadnano files that use unusual/new features, and so knowing what is causing things to break would help me tweak the input to avoid those features, or know what would need to be implemented in order to support the files.