before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter. but fortunately, fmt v10 brings the builtin
formatter for classes derived from `std::exception`. but before
switching to {fmt} v10, and after dropping `FMT_DEPRECATED_OSTREAM`
macro, we need to print out `std::runtime_error`. so far, we don't
have a shared place for formatter for `std::runtime_error`. so we
are addressing the needs on a case-by-case basis.
in this change, we just print it using `e.what()`. it's behavior
is identical to what we have now.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#17954