NAS-107302: Clarify two Periodic Snapshot error messages

- Rework dataset not found to be clear that a dataset or zvol was not found. This should solve multi-window/user situations where a dataset/zvol is deleted while the other window has it selected.
- Clarify the exclude message a little.

(cherry picked from commit 07aa0d7)

src/middlewared/middlewared/plugins/snapshot.py

@@ -294,13 +294,13 @@ async def _validate(self, data):
         if data['dataset'] not in (await self.middleware.call('pool.filesystem_choices')):
             verrors.add(
                 'dataset',
-                'Invalid ZFS dataset'
+                'ZFS dataset or zvol not found'
             )
 
         if not data['recursive'] and data['exclude']:
             verrors.add(
                 'exclude',
-                'Excluding datasets has no sense for non-recursive periodic snapshot tasks'
+                'Excluding datasets is not necessary for non-recursive periodic snapshot tasks'
             )
 
         for i, v in enumerate(data['exclude']):