CP-8547: Add button in XenCenter to initiate space reclamation on an SR: Changes following code review.

- Added check for null for sm.features in SR.SupportsTrim;
- Better error messages;
- Moved error messages from Messages to FriendlyNames;
- Removed the check for actions in progress from TrimSRCommand.CanExecute().

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Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>