pkg_add: do not emit error on installed packages

I have a large directory with packages that need to be installed during image build. Right now pkg-add(8) must be used with the force flag so that no errors are produced for already installed package *dependencies*. This also mutes real problems like checks for clashing files between packages... In contrast, pkg-install(8) neatly confirms the package is already installed and returns successful.

This fixes the issue while retaining the error behaviour for locked packages.