correct log and complete ut for error_monitor

[jlsong01]

What changes were proposed in this pull request?

code enhancement

Why are the changes needed?

1. make log output more clear, since in k8s world you can ONLY mark a worker NODE unschedulable, while not a POD.

2. complete the unit test for error monitor

Does this PR introduce any user-facing change?

no

How was this patch tested?

pre-commit test and unittest passed

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.14%. Comparing base (d2ea4a7) to head (e909a9c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
dlrover/python/master/monitor/error_monitor.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1347   +/-   ##
=======================================
  Coverage   81.13%   81.14%           
=======================================
  Files         231      231           
  Lines       21988    21989    +1     
=======================================
+ Hits        17840    17842    +2     
+ Misses       4148     4147    -1     

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