new: Merge error checks from add_files and add_files_recursive

new: Merge error checks from add_files and add_files_recursive

Previously, add_files_recursive could have been called on a symlink to
a non-directory.  Hence, calling it on a non-directory was not an
error, so a separate function, add_files, existed to fail loudly in
situations where the path had to be a directory.

With the new stat-ing logic, add_files_recursive is always called on
directories, so the separation of this logic is no longer necessary.
Hence, this patch moves the strict error checking previously done by
add_files into add_files_recursive.