Fix UniqueConstraintStressIT: Catch expected exception: Constraint violation on slave.

The constraint added by the master transaction will eventually get pulled by the slave
and a constraint violation due to a slave will be caught during the transaction building
phase instead of the commit phase, resulting in different end user exceptions.

A test case variation making this case more likely was added.
Merge pull request #3632 from martinfurmanski/2.2-unique-constraint-stress-it-slave-constraint-case

Fix UniqueConstraintStressIT: Catch expected exception: Constraint violation on slave.