0609cd676f
before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter.
in this change, `fmt::formatter<T>` is added for classes derived from
`cassandra_exception`, where `T` is the class type derived from
`cassandra_exception`.
this change is implemented to be backward compatible with {fmt} < 10.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>