This turned into a bigger change than I expected. While testing the input for the sample app, I tried a bunch of queries that threw schema_exception and eventually crashed. Part of the problem was that I deleted the copy constructor on schema_exception, but that's part of the interface for std::exception which is used by std::exception_ptr, and it ended up dropping the _errors member in the copy operation.

Once that was fixed, I noticed that higher levels were eating schema_exception along with std::exception and turning more specific error messages into "Unknown error" messages at that level of the query. I added catch (schema_exception &) blocks to each of those spots, and instead of turning it into a single generic error message it will either propagate the schema_exception or append its error messages to the list of errors that will later be rethrown in a schema_exception.