Inspect violated constraints

[taion]

cc @cancan101 @itajaja

This is obviously very rough and the final API will need to be more flexible, but the basic approach appears to work. Thoughts?

[cancan101]

I think one option would be to make this lookup be a set of utility function and then let the writer of the view decide how they want to do the mapping of the pg error to the api error.

[taion]

@sloria since you looked at this – is there an easy way to go from attribute name to data key, given a schema?

[sloria]

I think it'd be

attr_to_data_key = {
    field.attribute or field_name: field.data_key or field_name 
    for field_name, field in schema.fields.items()
}