Capture command error output

[yujinqiu]

➜  a git:(master) ✗ ERROR=`cp c d`
cp: c: No such file or directory
➜  a git:(master) ✗ echo $?
1
➜  a git:(master) ✗ echo $ERROR

➜  a git:(master) ✗ ERROR=`cp c d 2>&1`
➜  a git:(master) ✗ echo $?
1
➜  a git:(master) ✗ echo $ERROR
cp: c: No such file or directory

According above test suggests ERROR is always EMPTY string if we don't capture command error output

[codecov]

Codecov Report

Merging #927 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   75.52%   75.57%   +0.04%     
==========================================
  Files         206      206              
  Lines        9448     9448              
==========================================
+ Hits         7136     7140       +4     
+ Misses       1646     1642       -4     
  Partials      666      666

Impacted Files	Coverage Δ
transport/internet/kcp/dialer.go	77.35% <0%> (-7.55%)	⬇️
transport/internet/kcp/io.go	59.09% <0%> (-4.55%)	⬇️
common/log/logger.go	84.37% <0%> (-3.13%)	⬇️
common/buf/writer.go	70.32% <0%> (-2.2%)	⬇️
transport/internet/kcp/listener.go	87% <0%> (-2%)	⬇️
transport/internet/kcp/sending.go	87.11% <0%> (-1.34%)	⬇️
transport/internet/kcp/connection.go	72.85% <0%> (-0.56%)	⬇️
proxy/vmess/outbound/outbound.go	79.77% <0%> (+1.12%)	⬆️
proxy/freedom/freedom.go	81.25% <0%> (+1.25%)	⬆️
transport/internet/headers/http/http.go	81.56% <0%> (+2.12%)	⬆️
... and 2 more

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