check if core fields exist in the schema

If there is no core fields in the schema, no error message will be displayed on browser.
Scheming should probably fail loudly when required core fields are missing from the schema.

Start by tracing how Scheming validates schemas and how missing core fields reach the browser, since the issue names no files or tests. Confirm the behavior with a schema lacking required core fields; done means Scheming fails loudly with a clear error instead of silently showing no browser message.

Written by the indexing model from the issue text.