Enhancement: More user-friendly error message

[gogim1]

Search before asking

• I had searched in the issues and found no similar issues.

Motivation

I get two kinds of error messages when I exec kvrock2redis like this:

• if the data-dir in config file is an unexist path, it shows Failed to load config, err: NotFound:
• if the config file is not found, it shows Failed to load config, err: No such file or directory

I think it need a more accurate error message to distinguish this two cases.

Solution

Maybe the simpliest way to solve this problem is to append + data_dir after s.ToString():
https://github.com/apache/incubator-kvrocks/blob/c391953f92fc03b4d22e100a42024a898f3fae47/tools/kvrocks2redis/config.cc#L143

Are you willing to submit a PR?

• I'm willing to submit a PR!

[git-hulk]

This improvement makes sense to me