> "query failed" or "query succeeded, here's the data"

Blind SQL injection is a type where no error is produced, but some subtle signal can indicate success or failure. The most interesting one that I know about is where the presence of a successful injection was a normal looking response that was one byte longer than an unsuccessful injection. This was used to not only figure out the schema, but to fully exfiltrate the entire database.

There is nothing in the log on the server that indicates an error.

Most of the relatively introductory SQL injection exercises that I taught proceed without any knowledge of the schema.

This is why SQL injection is so insidious.