ldb tool : please make the error message when converting from universal compaction to level compaction more descriptive.

Currently, if someone tries to change compaction style to level compaction in order to export the database over the network, the message will read like this :

sh-5.1# ldb --db=/mnt/openethereum/xdai/db/3c648cbdc4c9333d/archive/db change_compaction_style --old_compaction_style=1 --new_compaction_style=0 --ignore_unknown_options --block_size=33554432 --compression_type=no --try_load_options --auto_compaction=true --block_size=33554432 --file_size=1572864
Failed: Convert from universal compaction to level compaction. Nothing to do.

This is misleading because the underlying issue isn’t tied to a specific database, but according to the source code, the real issue is https://github.com/facebook/rocksdb/blob/main/tools/ldb_cmd.cc#L2396 ldb doesn’t support it.

**The message should be changed for stating more clearly this isn’t supported by the `ldb` tool and isn’t specific to a database.**

[Relevant commit explaining the situation](https://reviews.facebook.net/D12603)