Fix a false negative for Rails/TransactionExitStatement

[Tietew]

#665 fixed a false positive when break is used in a block in transactions.
But it introduced a false negative on return or throw.

transaction do
  loop do
    break  # allowed
    return # should be detected
    throw  # should be detected
  end
end

This PR does not cover following false positive.
It's too complex to detect. And a bad code I believe (should be extracted to method).

transaction do
  catch :foo do
    throw :foo if condition1 # false positive -- just exit catch
    throw :bar if condition2 # true positive
  end
end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

This definitely advances the improvement step. Thank you!