query: change (debug-mode) field validation errors

Currently, the single assert just checks that the entire row is
consistent (true/false), and dumps the row and field definitions as an
accompanying message. This makes it very hard to understand what failed.

This patch changes this validation to show descriptive messages, which
makes it much faster in diagnosing invalid result.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>