Do not check error message

[hofnerb]

Test does not adhere to the guidance given in https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Writing-portable-packages:

Do not test the exact format of R messages (from R itself or from other
packages): They change, and they can be translated.

Packages have even tested the exact format of system error messages, which are platform-dependent and perhaps locale-dependent.

and thus results in an error:

 ── 1. Failure: write.bib (@test-toLatex.R#105) ────────────────────────────────
 `write.bib("nonexisting_pkg")` threw an error with unexpected message.
 Expected match: "package .* not found"
 Actual message: "there is no package called 'nonexisting_pkg'"