scalar-diagnose: add directory to archiver more gently

scalar-diagnose: add directory to archiver more gently

If a directory added to the 'scalar diagnose' archiver does not exist, warn
and return 0 from 'add_directory_to_archiver()' rather than failing with a
fatal error. This handles a failure edge case where the '.git/logs' has not
yet been created when running 'scalar diagnose', but extends to any
situation where a directory may be missing in the '.git' dir.

Now, when a directory is missing a warning is captured in the diagnostic
logs. This provides a user with more complete information than if 'scalar
diagnose' simply failed with an error.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>