make_entry_descriptor() and the two overloads of parse_path() used to signal
parse failures by throwing malformed_sstable_exception, which made parse_path()
expensive to use as a probe (e.g. to classify directory entries).
Change make_entry_descriptor() and both parse_path() overloads to return
std::expected<T, sstring>, where the sstring carries the error message on
failure, eliminating the exception overhead at probe call sites.
Call sites that previously caught malformed_sstable_exception to treat the
path as a non-SSTable file (utils/directories.cc, db/snapshot/backup_task.cc,
tools/scylla-sstable.cc) now check the expected result directly.
Call sites where a parse failure is a genuine error (sstable_directory.cc,
sstables.cc, tools/schema_loader.cc, tools/scylla-sstable.cc) re-throw
explicitly as malformed_sstable_exception using the error string, preserving
the existing error propagation behaviour.
c3daa6379c