Use the same error messages in the RPC code as the regular code

lib/browser/index.js

@@ -83,14 +83,14 @@ export default class Realm {
             if (typeof item == 'function') {
                 let schema = item.schema;
                 if (!schema || typeof schema != 'object') {
-                    throw new Error("Realm object constructor must have 'schema' property");
+                    throw new Error("Realm object constructor must have a 'schema' property.");
                 }
 
                 let {name, properties} = schema;
                 if (!name || typeof name != 'string') {
-                    throw new Error("Realm object schema must have 'name' property");
+                    throw new Error(`Failed to read ObjectSchema: name must be of type 'string', got (${typeof name})`);
                 } else if (!properties || typeof properties != 'object') {
-                    throw new Error("Realm object schema must have 'properties' property"); 
+                    throw new Error(`Failed to read ObjectSchema: properties must be of type 'object', got (${typeof properties})`);
                 }
 
                 schemas.splice(i, 1, schema);

lib/browser/objects.js

@@ -85,5 +85,5 @@ export function typeForConstructor(realmId, constructor) {
         }
     }
 
-    return null;
+    throw new Error("Constructor was not registered in the schema for this Realm")
 }