When set to false, no exceptions will be raised from the validator on
validation error. Instead, it will just return false from the respective
validator methods. This makes testing simpler, asserting exceptions is
clunky.
When true (default), the previous behaviour will remain: any validation
error will invoke on_internal_error(), resulting in either std::abort()
or an exception.
(cherry picked from commit f6511ca1b0)
9e7cd767dd