tests/secondary_index_test: Don't catch polymorphic exceptions by value
tests/secondary_index_test: Don't catch polymorphic exceptions by value

Don't slice exception by catching them by value. Instead of catching
by reference, use assert_that_failed().

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20180506153745.4512-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla