fix/refactor: fix propose doesn't handle SyncedError and reduce code duplication

[themanforfree]

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?)
  fix propose doesn't handle SyncedError
• what changes does this pull request make?
  return an Error instead of unreachable, and do some refactor to reduce code duplication
• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)
  no

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.09% 🎉

Comparison is base (9b747ec) 53.60% compared to head (0c47d3a) 53.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
+ Coverage   53.60%   53.69%   +0.09%     
==========================================
  Files         102      101       -1     
  Lines       15944    15913      -31     
  Branches    15944    15913      -31     
==========================================
- Hits         8546     8545       -1     
+ Misses       6878     6847      -31     
- Partials      520      521       +1     

Files Changed	Coverage Δ
curp/src/client.rs	5.16% <0.00%> (+0.01%)	⬆️
xline-client/src/clients/auth.rs	0.00% <0.00%> (ø)
xline-client/src/clients/kv.rs	0.00% <0.00%> (ø)
xline-client/src/clients/lease.rs	0.00% <0.00%> (ø)
xline-client/src/clients/lock.rs	0.00% <0.00%> (ø)
xline/src/client/mod.rs	0.00% <0.00%> (ø)
xline/src/server/auth_server.rs	0.00% <0.00%> (ø)
xline/src/server/command.rs	68.06% <0.00%> (-4.84%)	⬇️
xline/src/server/kv_server.rs	37.46% <0.00%> (+1.88%)	⬆️
xline/src/server/lease_server.rs	0.00% <0.00%> (ø)
... and 3 more

... and 3 files with indirect coverage changes

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