* Fixed how blob storage PutBlob errors are handled in content.Manager
In order to guarantee that all index entries have corresponding
pack blobs, we must ensure that `content.Manager.Flush` will
not succeed unless all pending writes have completed.
Added test that simulates various patterns of PutBlock failures and
ensures that data remains durable despite those, assuming all calls
to `WriteContent()` and `Flush()` are retried.
* addressed review feedback
d0e4d4f4cc