MINOR: [R] progress towards removing Assertthat dependency

[olivroy]

Rationale for this change

To get rid of assertthat dependency

What changes are included in this PR?

• progress towards removing assertthat dependency (replaced by rlang's type checkers)

Are these changes tested?

By existing tests.

Are there any user-facing changes?

No, only error messages standardized.

I have not been able to install the dev version locally, so I would need the workflows to be run, so that I could adjust the tests.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview