show a more descriptive error for weak Release files

If we can't work with the hashes we parsed from the Release file we
display now an error message if the Release file includes only weak
hashes instead of downloading the indexes and failing to verify them
with "Hash Sum mismatch" even through the hashes didn't mismatch (they
were just weak).

If for some (unlikely) reason we have got weak hashes only for
individual targets we will show a warning to this effect (again, befor
downloading and failing the index itself).

Closes: 806459

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