Throw specific exception instead of assert

I noticed that there are many assert in the programme which may cause levelDB to crash. May I know whether there are specific reasons we only assert instead of using specific exceptions?

For example, in `write_batch.cc`

```
void WriteBatchInternal::SetContents(WriteBatch* b, const Slice& contents) {
assert(contents.size() >= kHeader);
b->rep_.assign(contents.data(), contents.size());
}
```

Thank you.