ovsdb: Improve error message for duplicate uuid-name.

ovsdb: Improve error message for duplicate uuid-name.

ovsdb_execute_insert() tried to return a helpful error message when there
was a duplicate uuid-name, but ovsdb_execute() (its caller) makes any
parse error override a parse error.  Since ovsdb_execute_insert() would
skip parsing the row when the uuid-name was a duplicate, this meant that
the error actually reported would be that "row" was not allowed here, which
wasn't at all helpful (since "row" is in fact mandatory).

This commit clears up the problem by always retrieving the "row" member,
which required a small amount of refactoring, and adds a test.