The new error type `ErrBlobAlreadyExists` should be returned when we
try to put a blob using a blob.ID that already exists, and that is
somehow problematic for this operation. This error type may be
useful for other operations in the future.

This will be used to indicate that a PutBlob operation should fail if
it is attempting to modify a blob that already exists. We can think
of it also as "do not overwrite" or "create only if exists".

Expanded the function `VerifyStorage` to test both write paths
where DoNotRecreate is true and false.

For all storage types that do not (or do not yet) support idempotent
blob creates, throw an explicit error indicating so.

Was using the old name `RecreateIfExists`, changed to use
`DoNotRecreate`.

`ErrBlobAlready exists` -> `ErrBlobAlreadyExists`