Learning To Rank model upload fails generically

When uploading a model, using a not existent store or other incorrect parameters you get:

"error":{
"metadata":[
"error-class","org.apache.solr.common.SolrException",
"root-error-class","java.lang.ClassCastException"],
"msg":"org.apache.solr.ltr.model.ModelException: Model type does not exist org.apache.solr.ltr.model.LinearModel",
"code":400}}

In the response, logs don't help that much out of the box, I had to go for remote debugging and of course we don't want the generic user to do that.

Reason is in org/apache/solr/ltr/model/LTRScoringModel.java:111

try {
      // create an instance of the model
      model = solrResourceLoader.newInstance(
          className,
          LTRScoringModel.class,
          new String[0], // no sub packages
          new Class[] { String.class, List.class, List.class, String.class, List.class, Map.class },
          new Object[] { name, features, norms, featureStoreName, allFeatures, params });
      if (params != null) {
        SolrPluginUtils.invokeSetters(model, params.entrySet());
      }
    } catch (final Exception e) {
      throw new ModelException("Model type does not exist " + className, e);
    }

This happens when:

• use a not existent feature store
• use not existent feature
• use an integer instead of Double as a weight in a linear model

unless any objection, we should improve such message with the real one