refactor: execution error types

[bsbds]

Described in: #397

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Patch coverage: 10.25% and project coverage change: +0.10% 🎉

Comparison is base (0789ec3) 52.34% compared to head (879d3ec) 52.44%.

❗ Current head 879d3ec differs from pull request most recent head 54d7f8c. Consider uploading reports for the commit 54d7f8c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   52.34%   52.44%   +0.10%     
==========================================
  Files          97       98       +1     
  Lines       15166    15102      -64     
  Branches    15166    15102      -64     
==========================================
- Hits         7938     7920      -18     
+ Misses       6733     6686      -47     
- Partials      495      496       +1     

Files Changed	Coverage Δ
curp-external-api/src/cmd.rs	27.77% <0.00%> (ø)
curp/src/client.rs	5.14% <0.00%> (-0.08%)	⬇️
curp/src/error.rs	0.00% <0.00%> (ø)
curp/src/rpc/connect.rs	21.87% <0.00%> (ø)
curp/src/rpc/mod.rs	26.94% <0.00%> (+0.27%)	⬆️
...urp/src/server/cmd_worker/conflict_checked_mpmc.rs	85.08% <0.00%> (ø)
curp/src/server/curp_node.rs	24.30% <0.00%> (+0.10%)	⬆️
xline-client/src/error.rs	0.00% <ø> (ø)
xline/src/client/errors.rs	0.00% <ø> (ø)
xline/src/server/auth_server.rs	0.00% <0.00%> (ø)
... and 14 more

... and 2 files with indirect coverage changes

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[Phoenix500526]

I think this implementation is too invasive for the curp consensus. ProposeError::ExecutionError refers to errors gnenrated during the execution phase of CURP protocol. In fact, the client's command may not be turly executed during this phase. It will be executed during the after sync phase. Therefore, I don't think it's a good practice to include C::Error in ProposeError::ExecutionError. Can we instead combine the specific command's execution error and the specific command's response together? For example, distribute the ExecuteError to ER (CommandResponse) and ASR (SyncResponse)?

[bsbds]

I have pushed another refactored version @Phoenix500526 @rogercloud @iGxnon