[CI] Change default checkstyle severity to error

[kaijchen]

What changes were proposed in this pull request?

Change default checkstyle severity to error.

Why are the changes needed?

In common sense, "warning" is something can be ignored.
It is better to use "error" to mean something must be fixed.
And current CI workflow displays "Summary of failures" by filtering errors.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Example of checkstyle failure, click "Summary of failures" to see the filtered result.
https://github.com/kaijchen/incubator-uniffle/runs/7319512298?check_suite_focus=true

[codecov-commenter]

Codecov Report

Merging #57 (0beea7f) into master (aa02ee6) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #57   +/-   ##
=========================================
  Coverage     54.89%   54.89%           
  Complexity     1092     1092           
=========================================
  Files           146      146           
  Lines          7775     7775           
  Branches        749      749           
=========================================
  Hits           4268     4268           
  Misses         3270     3270           
  Partials        237      237           

Impacted Files	Coverage Δ
...storage/handler/impl/DataSkippableReadHandler.java	81.25% <0.00%> (-3.13%)	⬇️
...e/uniffle/server/storage/SingleStorageManager.java	67.21% <0.00%> (+1.63%)	⬆️

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[jerqi]

LGTM