chore: tune return msg

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Now errors are divided into 2 kinds

1. our handmade snafu error
2. external error, like std::io::ERROR

we now return the source error's message to client (e.g: mysql client, http response, etc.)

1. if source error is our snafu error, then {error} is returned, content of message should be #[snafu(display)]
2. if source error contains an external error, then {error}: {external_err} is returned for detail explanation.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

This pr should hopefully be the last one for #1854.

[codecov]

Codecov Report

Merging #2506 (37e2009) into develop (e352fb4) will decrease coverage by 0.36%.
Report is 3 commits behind head on develop.
The diff coverage is 85.41%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2506      +/-   ##
===========================================
- Coverage    85.25%   84.90%   -0.36%     
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  Files          729      729              
  Lines       115862   115914      +52     
===========================================
- Hits         98780    98414     -366     
- Misses       17082    17500     +418