pmb.helpers.repo.download: proper error for 404

Flush the progress bar displayed by the function before logging a warning message for 404 not found responses from the server. Without flushing, this message gets lost.

In addition to the warning, display a NOTE and ERROR at the end to explain what went wrong:

  [20:16:50] Update package index for x86_64 (4 file(s))
  [20:16:51] WARNING: file not found: http://mirror.postmarketos.org/postmarketos_get_404_test/edge/main/x86_64/APKINDEX.tar.gz
  [20:16:51] NOTE: check the [mirrors] section in 'pmbootstrap config'
  [20:16:51] ERROR: getting APKINDEX from binary package mirror failed!

Without handling this properly, pmbootstrap would fail slightly later with a much less useful error:

  [15:14:28] ERROR: expected str, bytes or os.PathLike object, not NoneType

Fixes: #2424 (closed)