Use check installed not require namespace for better UX

[B-Nilson]

See issue #515

Did a find and replace within R/ for requireNamespace, then cleaned up related code.

Notably:

• sapply(pkgs, ...) implementations are no longer needed as rlang::check_installed handles multiple packages
• custom error messages no longer needed as rlang::check_installed is more clear
• quietly = TRUE no longer needed as rlang::check_installed is quiet unless throwing an error