-// PutBlock stores the BLOCK on the first Keep volume with free space.
-// A failure code is returned to the user only if all volumes fail.
-//
-// On success, PutBlock returns nil.
-// On failure, it returns a KeepError with one of the following codes:
-//
-// 500 Collision
-// A different block with the same hash already exists on this
-// Keep server.
-// 422 MD5Fail
-// The MD5 hash of the BLOCK does not match the argument HASH.
-// 503 Full
-// There was not enough space left in any Keep volume to store
-// the object.
-// 500 Fail
-// The object could not be stored for some other reason (e.g.
-// all writes failed). The text of the error message should
-// provide as much detail as possible.