Add error handling in jsonschema

[acmiyaguchi]

https://github.com/acmiyaguchi/jsonschema-transpiler/blob/d39b9e7cede239cbe1e83043d451a00de861e4d3/src/jsonschema.rs#L113

The transformation from jsonschema into the ast should provide better error handling for schemas that are not valid. There are a few places where the code will panic instead of propagating useful information up. A Result<T, E> type is appropriate since error messages can help users figure out why their schemas are bad.