Clean up RemoveImage error handling

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Switch to fmt.Println

The println builtin is not meant for production code.  Use fmt.Println
instead.  This fixes missing input in capstan_test.go test cases.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Test cases for more commands

Now that the test harness captures is able to capture capstan output,
add more test cases.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>