Nice error message if go.mod does not contain dependencies

[johanneskastl]

I would propose that the service errors out with a useful message instead of this if the go.mod file does not have any dependencies in it:

Example: https://github.com/minio/certgen/blob/main/go.mod

module github.com/minio/certgen

go 1.21

INFO:obs-service-go_modules:Vendor go.mod dependencies to vendor.tar.gz
Traceback (most recent call last):
  File "/usr/lib/obs/service/go_modules", line 322, in <module>
    main()
  File "/usr/lib/obs/service/go_modules", line 315, in main
    new_archive.add_files(vendor_dir)
  File "/usr/lib/python3.11/site-packages/libarchive/write.py", line 84, in add_files
    r = read_next_header2(read_p, entry_p)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/libarchive/ffi.py", line 98, in check_int
    raise archive_error(args[0], retcode)
libarchive.exception.ArchiveError: vendor: Cannot stat (errno=2, retcode=-25, archive_p=94160168216544)

(The output was achieved when using the modified service from #51, otherwise it never reaches the actual go step)