Unify usage of storage error exception type

- Instead of formatting the exception by creating
  a storage error instance we can just let the already
  included cause make this formatting occur.
- Handle cases in impl_dir which were not catching and
  raising storage errors.
- Unify usage of 'backend internal error' messaging.
- Pass the cause to storage errors where applicable.

Change-Id: Ieb14256b202ccbfa3a96f68fa35db7e40f92114d
Use the stop watch utility instead of custom timing

Instead of retaining a internal expiration and timing
functionality in the remote_task, just use the stop
watch helper class already existing in the misc utility
module.

Change-Id: Ie50c79ee6a5cff16c52a0fae0a65544a0d4202e1
Merge "Use the stop watch utility instead of custom timing"
Merge "Unify usage of storage error exception type"