Comparator::Compare() has no way to signal corruption

The API for `leveldb::Comparator::Compare()` has no provision for failure. Failure is a reasonable response when one of the inputs is not a valid application key. Assuming no bugs in LevelDB, this can happen if on-disk keys are corrupted.

Example: suppose the on-disk key is a pair of strings, encoded as varint(length of string 1) | string1 | string 2. An input of `0x05 0x01` is then invalid -- s1's length is 5, but the key only has one more byte.