fix(jsii-spec): show deep validation errors (#452)

Author: rix0rrr

packages/jsii-spec/lib/validate-assembly.ts

@@ -6,7 +6,8 @@ export const schema: jsonschema.Schema = require('../schema/jsii-spec.schema.jso
 
 export function validateAssembly(obj: any): Assembly {
     const validator = new jsonschema.Validator();
-    const result = validator.validate(obj, schema);
+    validator.addSchema(schema); // For definitions
+    const result = validator.validate(obj, schema, { nestedErrors: true } as any); // nestedErrors does exist but is not in the TypeScript definitions
     if (result.valid) { return obj; }
     throw new Error(`Invalid assembly:\n${result}`);
-}
+}