Use a different error message in --diff to signal that module was not found

If a module or path is not found it will show this error

```
$ twistedchecker bla/bla
************* Module bla/bla
F0001:  1,0: No module named bla/bla

$ twistedchecker bla.bla
************* Module bla.bla
F0001:  1,0: No module named bla.bla
```

but when I run it with diff, it will show the same error and then the diff code will not detect is as a new error
