do not discard valid data in buffers when in tolerant mode

do not discard valid data in buffers when in tolerant mode

Both decryption and decompression will fail on the first error
and ignore any results of earlier passes. In normal operation,
the hard failures are desirable to indicate a bad backup set.

However, in tolerant / recovery mode the error handling is closer
to the opposite extreme: we want to retrieve every last byte that
made it through the various layers and only skip over the parts
that cannot be interpreted at all.