Make sure `total_count` and `errors_count` methods in `CodequalityReportsComparer` are implemented correctly

Problem

In my understanding, total_count is for checking all entries in the head report, and errors_count is for checking all error entries in the head report. Now, CodequalityReports can have only error type of entries, these methods should be identical in the CodequalityReportsComparer domain. However, currently it's inconsistent.

We should make sure what the pusposes/intentions on these methods.

cc @morefice

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The following discussion from !48385 (merged) should be addressed:

• @shinya.maeda started a discussion:

  The same question with the Accessibility Report bug goes to this method. It seems we simply can do:

  def total_count
    head_report.degradations_count
  end
  alias :errors_count :total_count