Fix `ocfl.storage_root` to use validation error logging and codes linking to spec

[zimeon]

ocfl.storage_root uses a plain-text StorageRootException for many validation errors defined by the specification that should be reported using ValidationLogger. The use a a separate .structure_error property also seems unhelpful and should be replaced with ValidationLogger.

Also need to move various directories in extra_fixtures to the extra_fixtures/1.0 directory so we can add new fixtures for 1.1 and 2.0.