The `leveldb::Status` class logs the filename of corrupted files, which might be useful when looking at error reports from usres. In theory this is already logged via the `LogPrintf()` statement in `HandleError()`, but that may not always be close to where the final error message is logged, e.g. see https://github.com/bitcoin/bitcoin/issues/11355#issuecomment-340340542 where the log trace provided by the user does not contain that information (and other user comments in the same issue).

This also adds a log message instructing the user to run the process with `-debug=leveldb`, which provides much more verbose error messages about LevelDB internals. This may not really help much, but improving the error messages here can't hurt.