Made option value assignment use transactions.

Made option value assignment use transactions.

It's now more predictable what will happen when option assignment fails:
if assignment of a set of values fails, the whole set is rolled back.
Within a set, there are still cases where the state may be different
after error recovery than it was before the erroneous values were added
(if value conversion may create more than one value from a single
value), but these could also be handled if they become issues.