Fix exception handling on test_delete_nonexistent_volume

test_delete_nonexistent_volume wants to check the exception would be
received when nonexistent volume was specified. But currently, this
test case checks the exception would be received when nonexistent
metadata was specified.

Change-Id: Ia110618d5c658551a1826d495656954be02f4326
Merge "Fix exception handling on test_delete_nonexistent_volume"