sstables: Fix string formatting for exception messages in m_format_read_helpers.
sstables: Fix string formatting for exception messages in m_format_read_helpers.

Before this fix, the code was a potential undefined behaviour and crash
because it would add a large value to a const char* and try to create a
std::string out of it.

Signed-off-by: Vladimir Krivopalov <dc9186a06078733915a6fcbab34e59120be2b484@scylladb.com>

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