live: validate file existence and checksum for unicast cache

after copying file to cache, validate that file exists in cache and checksum
correct.

src/live/ogOperations.py

@@ -208,6 +208,11 @@ class OgLiveOperations:
             if (not os.path.exists(image_path) or
                 not tip_check_csum(repo, name)):
                 self._copy_image_to_cache(name)
+
+            if (not os.path.exists(image_path)):
+                raise OgError(f'could not find {image_path} in cache')
+            if (not tip_check_csum(repo, name)):
+                raise OgError(f'Failed to validate checksum for {name}.img')
         else:
             if os.access(f'/opt/opengnsys/images', os.R_OK) == False:
                 raise OgError('Cannot access /opt/opengnsys/images in read mode, check permissions')